Introduction: DIY Hi-Fi: Class a Hybrid Headphone Amp

Picture of DIY Hi-Fi: Class a Hybrid Headphone Amp

FORWARD::::::: This project was born out of a couple things. One was a need for an amp to drive some higher impedance headphones I had just bought. (The Grado SR125i headphone is great, by the way.) Also, since I have began my freshman year at Boston University, and could not resist using the Singh Imagineering Lab- a workshop only for College of Engineering students that allows you to work on pretty much anything. They also provide basic materials and components, as well as lathes, NC mills, soldering stations an electrical equipment, and a 3D printer. This is is just the tip of the ice burg though.

Another reason for the project was an assignment from another class where I had to build something for my own enjoyment and benefit, but just had t keep a running journal on how the project went. This Instructable serves as my journal. Because of this, you will notice it is written as such, with running updates on aspects of the build and a more personal touch to the process than a simple step-by-step Instructable. 

So. Let's get started.

I chose a vacuum tube amplifier as opposed to sold state. The reasons to choose tubes are as follows:
-They sound better (this might be slightly subjective. But it is still true.)
-They glow 
-They are just cooler
-They glow
The circuit I used was chosen for its small part count, it ability to be run at 12 volts (massively simplifying the circuit and the parts needed), it's lack of needing an output transformer, and its relative efficiency. After a little browsing on Google, I found what I thought seemed to be very viable circuit. The circuit itself is highlighted in the next step.

Step 1: The Circuit

Picture of The Circuit

The circuit I chose to use is the circuit seen here. The reason for this is as follows:
-The circuit is simple. It uses only one twin triode tube. This allows the circuit to amplify both channels (left and right) with one tube. It also uses MOSFETs as the followers for the tubes. Tube followers require the use of an output transformer- this bites severely into the efficiency and size of the circuit (and makes it more expensive)
-The circuit uses an LM317 as the constant current source. This means that less energy is dissipated through it than it would be if a resistor was used to set the current, making the circuit more efficient.
-The 12AU7 tube used uses 12.6 volts as the heater voltage, and can be operated on a plate voltage as low as 12 volts. Operating at this low of a current can affect the performance slightly. But its still effective enough for what we're doing here.

ALL CREDIT FOR THE ORIGINAL CIRCUIT GOES TO ROGERS GOMEZ AT DIYAUDIOPROJECTS.COM.
NOTE:::::: Going the link provided has much more technical information and some assembly instructions of the circuit itself, including setting bias of the tubes and such. For this reason I will turn over much of the assembly instructions to that page. It also has the files for the circuit in CAD and images for the PCB. I don't want to cut into his page views, so this Instructable is  meant to be an addendum to his project and show my take, as well as provide some files to make a 3D model for the enclosure. 

I modified the circuit slightly from the original on the website. The modifications include a 1000uF capacitor on the power input to reduce the chance of noise as well as .22uF capacitors on the audio input for the same reason. I also added a switchable high cut-off, since the Grados can be a little bright when listening at high volumes to certain types of music. I have to say though, I barely ever use this function. I find the sound to be great as is. 

Step 2: Parts

There is a list of parts on the link to the original project. However, I am going to provide my modified list of parts that I used. The parts include some accessories, such as a battery and line out cable for my iPod.

When it came to ordering parts, it was apparent that I would have to order from multiple sources to track down the parts I really needed. Trying to keep a tab on all the parts, including the extra parts for the high cut and such, turned out to be a challenge. I had ordered parts for projects like this before, so I knew about the struggles I could face and the time consuming parts ordering came as no surprise. Ordering parts can be a hassle the first couple times, since there are often more than one component that will fullfill your needs, and choosing can be difficult. Sometimes the opposite happens and you cant find exactly what you need so you need to improvise. Making my way  through this struggle has become second nature at this point, but to make things easier on you I have included a complete parts list. 

Some of the parts I took from the Imagineering Lab to cut down on cost. These parts, however, are generic type parts that can be ordered just about anywhere. 

The parts list is: 

# --- Part #      --- Description from website and unit price

Parts Express:
2 --- 002-2.2K --- 2.2K Ohm 1/2W Flameproof Resistor 10 Pcs.  $0.75 
2 --- LM317T --- LM317T Variable Voltage Regulator TO-220  $2.25  
1 --- 230-112 --- FiiO L10 3.5mm Dock Cable for iPod iPhone iPad 20"  $8.99  
1 --- 365-008 --- Grip Tools 38020 12V Automatic Battery Float Charger  $14.50 
1 ---140-364 --- Power-Sonic PS-1250F2 Sealed Lead Acid Battery 12V 5Ah  $15.33

PartsConnexion:
1 --- ALPS-72497 --- Blue Velvet - STEREO Conductive Plastic Potentiometer, audio/log taper $19.95
2 --- ARCOL-74534 --- 10R, 1/2 Watt, 5%, Carbon Comp, Tinned Copper Leads $0.49
2--- ARCOL-75030 --- 150R, 1/4 Watt, 5%, CarbonComp, Tinned Copper Leads $0.33
2 --- ARCOL-75039 --- 1K, 1/4 Watt, 5%, CarbonComp, Tinned Copper Leads $0.33
2 --- ARCOL-75047 --- 4K7, 1/4 Watt, 5%, CarbonComp, Tinned Copper Leads $0.33
2 --- ARCOL-75063 --- 100K, 1/4 Watt, 5%, CarbonComp, Tinned Copper Leads $0.33
2 --- AXON-73720 --- 2.4uF 250V 5% TRUE CAPACITOR $1.43
1 --- SOCKET-59006 --- 9 PIN CER-PCB GZC9-B-GLD TUBE SOCK $2.95
2 --- NICHICON-72601 --- KZ 100uF 25V 10x16 $0.47
2 --- NICHICON-72610 --- KZ 470uF 25V 16x25 $0.85 
1 --- TUBEEH-62013 --- 12AU7 / ECC82 Dual Triode, 9 pin, Original Box, Russia, $10.95

Radioshack:
2 --- 276-2072 --- IRF510 Mosfet $2.49
2 --- 274-246 --- 1/8" Stereo panel mount jack $2.99
1 --- 274-1582 --- DC Power jack size M $3.99
1 --- 274-1569 ---DC power plug size M $3.49
 

This list does not include the .22 uf (x2) mylar cap for the filter on the input or the .47 uf (x2) mylar cap for the high cut. It also includes 4.7 kohm resistors which can be used in place of the biasing potentiometers from Radioshack. I recommend using the biasing pots because the circuit operates best when biased correctly and this is not guaranteed to happen with a fixed resistance of 4.7 kohm.
Also, a note about the switches: You need a single pole switch for the power but a dual pole switch for the high cut, These are available at Radioshack, but can be ordered from any of the above place for around the same price. You might want to choose your switch depending on which you like best.
And one final note: You should purchase a knob for the volume pot. I got mine through Radioshack, but again you can get them just about anywhere so shop around and choose the one you like best. 
And another final note: This is the part number for the perf board I used through Radioshack. This is the one the 3D enclosure file is sized for. It is not the exact same dimensions as the PCB file found on the original project page I have linked to. So keep this in mind when deciding the path for your project. 

Step 3: Enclosure Assembly

Attached are files to allow you to print up an enclosure like I used. It is sized so the proto-board that all the components are mounted to would snap into place. Also, as you can see, the base is a separate piece. The reason for this is that I wanted the enclosure to be removable from the base in case of repair or modification in the future.
The pieces are in inches, but can be scaled to metric pretty easily. The base fits tightly over the enclosure itself so that glue is not required. I ended up having to file and reshape the corners of the enclosure to match the base better due to the warped corners. Otherwise, the base would bend.  

When printing the enclosure, I realized just how awesome 3D printers are. When they are working. The printer in the Imagnieering lab is a Makerbot Replicator. (the day I typed this they unboxed a 2x) It works great, minus the small issues with clogged ejectors, unlevel build plates, random glitches when slicing... but those are usually few and far between. The major problem I kept having was the piece warping while printing. The build plate temperature was adjusted, the ejector temp adjusted, infill adjusted, print speed.... Eventually I got most of the parts straight enough that it worked. The corners of the enclosure itself on the bottom were still a little warped, but they are concealed by the base, which printed straight. Overall, I am not disappointed with the final result, though relieved that it is done and I don't have to worry about printing anymore. 

Some may look at the enclosure and realize there is no shielding. This is true. But i have found that the lack of shielding has not been that large of an issue. Problems only develop when a cell phone is placed next to the amp or a live power cord draped across it. But because of the battery power the circuit is very well isolated from interference. I have not had any problems with it.

If you plan on using this with a power adapter or in an environment prone to RF interference, I would recommend adding shielding to the enclosure or using a metal enclosure. In Boston it is just fine enough that I don't need the shielding, though I do find that sometimes when I place my amp near a large metal object like a fridge with the volume up all the way and no signal source connected (an unusual set of circumstances)  I can pull in radio stations quite clearly. It's an interesting trick, but has not presented any problems yet. 

Step 4: Time to Build!

Picture of Time to Build!

Here I am going to provide the pictures from my build, as well as tips and procedures specific to the build I did. But, I am going to turn over a lot of the build instructions to the link I referenced earlier. The reason for this is that there is no reason for me to provide the information complied by someone else and cut into their page views. I want to honor the property that is the original circuit designer's. Besides,  the build guide Rogers complied is excellent. 

Just some pointers specific to my project:

In reagrds to the circuit:
-SOLDER WIRES TO SWITCHES AND JACKS BEFORE INSTALLING  IN THE ENCLOSURE!!!!!!!!!!! The jacks and switches are meant to be glued into place to secure them. If you forget to solder your wires to the jack and have already glued it in place it can led to some awkward and difficult soldering. Save yourself some grief and think through your steps first or even make a list of steps that you will take
-The perf board (obviously) does not have traces like a PCB would. This means you have to run the traces yourself. A good way to do this is to not cut the leads off of the components when soldered in and bend them to where the connections need to be.
-When doing point to point wiring like is done here make sure there is a firm mechanical connection between electrical contacts before applying solder
-Do not overheat the components (common knowledge)
-Make sure to watch the clearance between the volume pot and the connections on the perf board above it so that everything will fit together properly.
-The holes for the pins for the tube socket will need to be enlarged. This might remove the pad entirely but you can bend the socket pins over to tough another pad.
-The capacitors for filtering and high cut are soldered to their respective jacks as opposed to the circuit board itself

In regards to the enclosure:
-ABS melts. Careful soldering
-The holes for the components might need to be filed out slightly larger in order for the components to fit

Step 5: Use It!

Picture of Use It!

So, this amp can be used with any source that can be plugged into the 1/8" input jack. This project was intended to yield a high quality amplifier to take the place of the headphone output on my iPod. With my amp I often use a Fiio adapter pictured to hook the line output from my iPod into the amp. By bypassing the internal volume control circuit and taking the signal directly from the internal DAC the sound quality is dramatically improved. By combining this with the use of lossless file encoding (Apple's ALAC or the longtime standard FLAC) the difference is extraordinary over the normal headphone output. Taking these steps are a requirement to get the most out of a good pair of headphones.

When using the amp with some source with volume control, make sure to have the volume of the source at about half volume (assuming its output is intended for headphones) 

There is no battery charge monitoring built in, so you will have to monitor battery voltage with a meter or just wait until things start sounding funny to know the battery is dead. Or, just develop a standard charging cycle of overnight every 2 days or something. 

As another note, don't get thrown off by the heat of the circuit. The MOSFETs and LM317s will get hot to the touch but not dangerously so. Also, the tube needs heat to operate properly so don't panic about it either if its warm. 

One other thing worth mentioning- Make sure you plug in the power supply and turn the amp on before anything else (source or headphones) are plugged in. Also, before listening it is good practice to give the tube time to warm up. This will only take a minute.

Step 6: Final Notes

Picture of Final Notes

Overall I found this a satisfying build. Being a budding audiophile, building your own equipment and actually having it sound great is very rewarding. While it was an expensive project for a class, it was not something that was built for the class; it was built for me to use regularly and get credit for the class at the same time. It was just a nice coincidence. 

In the future this amp might see slight modifications. Namely, a power adapter at some point and an enclosure redesign. I like the simple aesthetics of the enclosure now, but would like to add some shielding so I could potentially remove the filtering caps on the signal line. I would also like to add some sort of cage around the tube to make the amplifier somewhat portable. (Portable being a relative term for something that runs off of a lead acid battery) Also, the battery would somehow be integrated into the enclosure so it would be one self contained unit. 

I also found out, listening with this amp, a couple disappointing things:
-MP3 files suck
-Earbuds suck
The added clarity of using this amp over the iPod's internal amp led to me realizing how poorly MP3 files sound when it comes to low frequency reproduction and transient response. This was previously not detectable because the poor quality of the iPod amp masked the ability for a lossless file to be played 'losslessly'. Now that I have high fidelity sound hardware, the data source needs to be updated to match.
And now that I have a good source and amp, I can now easily tell the difference in clarity and accuracy between my Grados and Klipsch S4 earbuds (Which as earbuds go are very good).

Well, that concludes this journal entry/Instructable on my vacuum tube headphone amp. I hope you enjoyed it and found it informative and maybe has inspired you to take a second look into how you listen to music. There is truly a lot to be gained by improving the fidelity of your music. During the presentation of my amp to my class, my classmates were surprised at the clarity and how good the music sounded. Many of them were listening to songs that they were familiar with but to them it seemed like a whole new experience.  

Comments

Velociraptor420 (author)2017-04-18

Hello, Can I replace the headphones with a normal speaker and if yes, what power would the speaker need to be?

maresaldeflota (author)2017-02-04

Hello, i have a problem with the 10ohm resistors, they are burning what is wrong? I do all the circuit good no shorts, put togeter all parts but the 10 ohm r burns, please help me!

tenbucc2 (author)2014-03-01

Is there any possible way to recreate this using a 5volt power supply (I'm specifically thinking about trying to power this from USB), or would the amp be too underpowered?

if you want to power it from your pc, take 12V from a Molex, SATA or floppy connector.

kschmidt2 (author)tenbucc22014-10-06

If you're wanting to hook this up to a desktop computer you could always feed one of the HDD power connectors out through the back of the case or something, there's 12volts and a lot of amps too (Don't remember exactly how many)

JoeBeau (author)tenbucc22014-03-04

Unfortunately, no. The amp requires a minimum of 12 volts to operate. Some tube purists might say 12 is a little too low. USB provides 5 volts. And USB also does not supply enough current to allow a boost converter or something similar to get to 12 volts. So short answer is no, it would not be possible.

gussmith001 (author)2016-12-03

Awesome! I would like to stress the most important reason is that it GLOWS!!

R4MP4G3RXD (author)2016-04-03

Firstly it didn't work for me.. I didn't see my error and I was certain that everything was good... I even had my pops look at it to try and figure out what was wrong, and when I realized, after 2 days, that I wired the tube backwards.. :\
Luckily the tube survived and now it's working pretty good! :D
Now I'm just having trouble finding a good power supply for the damn thing, I might just end up using a 12v battery. :P

RezaH13 (author)R4MP4G3RXD2016-04-22

Use LM317 based adjustable power supply to provide constant an accurate 12.6 volt power and enjoy it!
the schematic is very simple and easy to build :)

KarlB26 (author)2016-02-04

So i have an interesting problem, i assembled the entire thing, but the irf532 that i power the heater off appears to recieve no voltage at all, the one on the other side is, everything is connected correctly, and considering its jist a wire i cannot think what the issue could be other than a busted mosfet. Any other ideas?

Thanks

R4MP4G3RXD (author)KarlB262016-04-03

Probably not connected properly. I had the same problem, and then I looked at the datasheet for the tube and realized I had accidentally wired in my tube inverted :\
Luckily the tube didn't brake, so I fixed my circuit and had a working tube amp.
The tubes pins, from the slot increase cw. So if you have the tubes' slot facing up it goes to the right.
HOWEVER if this is not your problem, and you are certain that you have wired everything correctly, then try removing the tube and try seeing if it draws current that way, if it doesn't then there is something wrong with the lm317. If it does, and you have a properly wired circuit than it could be a faulty tube.

ddelrio (author)2015-08-14

In my prototype, when I disconnect the power supply (12V Battery) occur a very powerful, unpleasant sound in the headphones. Any tips to remove that annoying noise?

kschmidt2 (author)ddelrio2016-03-25

Big Capacitor might help, either that or look up speaker protection circuits.

AzkaA2 (author)2015-10-23

I just finished my amp. and when I tried my amp, i found the volume so low and so much distortion. I already followed all direction and double checked my schematics. and I found no mistake. Did I miss something guys? thanks..

kschmidt2 (author)AzkaA22016-03-25

You might have the Mosfets and the power regulators backwards, I know I did. and mine sounded a bit like that when I did.

principle made it! (author)2015-11-22

Nice project i've not had time to case mine yet still testing, will replace the film caps with something a little better but... Great sound had trouble with noise on a lot of SMPS so i built a linear supply to go with it. The PSU is really overkill it was just the parts I had on hand.

Thanks

kschmidt2 (author)principle2016-03-25

This is very clean, I like it, and better to have to much amperage than not enough, I just used a 1.5amp external HDD power brick, it distorts a little when at full volume, so you've probably hit a winner with such a big power supply.

kschmidt2 made it! (author)2016-03-25

I made Myself one of these, only I used an IRF532 instead of an IRF510, I'd like to think it's the reason I have such depth to the sound and very hearty lows, it's an amazing full sound that no other amplifier I have can reproduce, that is if it's plugged into my MP3 player, Being a Cowon it has high quality DAC's and amps inbuilt, os this is just an extension to the amazing, and the Presonus HD7's finish it all. I't amazing, I rarely have it up past 3. Mounted the entire thing with old screws from a mac G5 case to space out the components from the tin, had to earth it because the metal case put a buzz up it otherwise, and I had to put on heatsinks because it just go too hot feeling, and the smell of burning circuit board was around. Mounted it in an old Krazy Bones lunchbox, I'm planning on putting a cage over the Tube, and I might end up painting it like a Vault-tec Lunch box from Fallout.

Thanks for the excellent Instructable, between this and the original site I was able to make a device that will follow me in life wherever I go.

Encom21 (author)2016-01-24

Would a 12AX7 tube would work for this project?

robot797 (author)Encom212016-01-27

yes

all double triodes will work
you only need to adjust the fillament connections

akristoffer (author)2013-12-04

Thanks for the swift response. My plan is to build a little dock for a phone. It's gotta look cool (which your amp does), sound good (again yours seems right), accept input from an array of different sources instead of being device specific, and be relatively cheap. I'll likely include a headphone jack as well as speakers, but speakers will be the primary output. Portable enough to move it into another room or pack for a trip, loud enough for maybe 1 or 4 people in a small to medium sized room. So I think this project is suitable after a few adjustments. And I think you covered everything. I hoped to cut cost by removing the battery, since it's unnecessary for my plan anyway. But it sounds like the speakers might bump the cost back up. If I remember right, screwing up the impedance will damage the speakers with a tube amp but damage the amp it it's solid state, though that might not be correct. If...when I build it, I'd rather screw up the speakers if I make a mistake instead if damaging the freshly built amp. Time to do some shopping I guess.

JoeBeau (author)akristoffer2013-12-04

parts-express.com is a good resource for anything audio related. It sounds like you are after a smaller, portable speaker system. Here is a good driver to check out:
http://www.parts-express.com/faitalpro-4fe35-4-professional-full-range-woofer-16-ohm--294-1125
It is on the smaller side. If you want something larger i would check out some of the pro-audio drivers Parts Express has. Something with higher impedance and efficient. The driver I linked to is full range, simplifying the design and making it cheaper now that there is no crossover or tweeter. Plus, with a well-designed enclosure, bass down to 56 hertz which is pretty good from a 4" speaker.
With the mosfet followers, as opposed to tube followers, I wouldn't be too worried about damaging anything immediately. You would see signs of distress (excess heat, distortion) before anything goes bad.

Encom21 (author)JoeBeau2016-01-24

I would like to know if the circuit would allow 12AX7 tubes as well

jimustanguitar (author)2015-09-27

Did you edit the schematic in the image to include the input potentiometer and the different trim pot (10k vs 50k) or has Roger posted multiple versions of the design? The schematic that I find on DIY audio is different in those two places.

Awesome work, BTW. Gearing up for my own build now!

riven67 (author)2015-09-11

Hi great project ! but I have a question that is very noobish, in every one of these headphone amp projects I have found no one shows in the schematic or pics how the rca jacks or the 3.5mm headphone jack is properly connected. specifically how it is grounded. Do you use the ground on the PCB as ground for these also? never having built any electronics these really basic things are unknown. can anyone comment and help me out, I'm thinking the grounding issue is causing the lack of good signal and static.

ddelrio (author)2015-08-14

jzqye made it! (author)2015-07-30

Great instructables guide. It was super easy to follow. We just finished the pcb version of the circuit. Moving from protoboard to pcb got rid of a lot of the noise and we added switches to bypass the amp and an led circuit just for swag.

fhar1 made it! (author)2015-07-15

Thanks for the instructable.

I used different approach to make this project.

I made the PCB board using the eagle circuit board file provided by Roger Gomez on Diyaudioprojects dot com.

To make the PCB, I used toner transfer method. Instead of using the well known glossy magazine paper as toner transfer medium(failed attempt as shown in photo), I used vinyl sheets to do the job. Vinyl is very cheap, about 1US dollars per square meter. 99.9% toner are transfered from vinyl to my copper board, this is very important for the success of the project. Remember to clean the copper board before toner transfer.

For etching, I don't want to deal with nasty chemicals, I used Vinegar + hydrogen Preoxide + salt as etching solution(see the household chemicals I used in the photo) , and used a sponge to scrube the copper board. The whole etching took about 5 minutes, not much bad smell or none at all is produced during the progress. A blue color copper acetate (poisonous) is by product.

After etching, use sandpaper to remove the remaining toner.

When it comes to drilling, I didn't buy a press drill. I bought a few drill bits of 0.8mm( for resistors), 1.0mm (for caps with thicker legs), 1.2mm( for the tube socket) .Using a toy drill made by Tamiya, I managed to drill the holes fairly quickly.

Soldering is easy with the PCB on hand.

Yeah, I have ordered all my parts from Taobao. Price is lower than mouser, radio shack.

Resistors 100K, 4.7k, 1k, 150 ohm 1/4w are sold at $1RMB for 100 pieces
resistors 10 ohm (1/2w) $7 RMB for 100pieces
LM317 - $4 RMB for 5pieces
IRF510 - $9 RMB for 5 pieces
2.2uf 16v non polar cap - $6RMB for two
100uf 16v cap - $8RMB for two
470uf 16v cap $2RMB for two
9pin tube socket $5RMB each
DC socket $1RMB each
Alps twin pot 50k $13RMB each
3.5mm audio socket $10RMB for Two
3296 variable pot $2RMB for both
copper board 5X7CM $1RMB

Total: 72RMB. which is around 12USD for all the components
( shipping charge from china to HK not shown)



GiwrgosP (author)2015-06-23

Im about to make this project though, i cant find 100k 1/4watt 5%, will try to some local store. The thing is i dont really care about it being portable and since tubes need to stay warm to achieve better quality i wonder if there is a power supply for this amplifier. Im using 220v AC. Thanks in advance will be waiting for a reply!

pvalyk (author)GiwrgosP2015-07-07

Hey there. I read on a forum that resistor values are not fixed. You can go higher and use from 100k to 1M.

Also, you can power it with a wall charger rated between 12v - 13v. Note that the power supply may add noise to your amp. I that chase, add 2200uF polarized capacitor between them

NateDemon (author)2014-11-25

I am new to audio amplification and I have recreated your headphones. However, I am finding it sounds rather quiet. I am not using the IPod headphone jack that you are using, and I am using a 100K Pot to for volume control. What would you suggest I do to increase the output volume?

JoeBeau (author)NateDemon2014-11-25

From what you describe, I would think that the different parts are not to blame for the issues you are having. Rather, there is probably a mistake in the construction or a faulty component. I would start by checking for bad solder joints; sometimes going through and reheating all joints to make sure the solder flows properly into the joint will fix a problem like you are having. But I would also recommend double checking your assembly against a schematic to make sure everything is correct

Nighter3D (author)2014-09-02

Heh thought the schematic looked a bit familiar.

It is indeed a great little project to start off from. Currently i'm experimenting with the mosfet output having been replaced with a OPA2134 Audio-amp for active filtering possibilities. currently waiting for some proper Tube dampers and precision pots for the bias resistors for further experimentation and tweaking.

The only thing that has me worried about this design is you 0.22uf RC High-pass filter. High-pass filters rely on the characteristic of both the capacitor and resistor. The potentiometer is in this case the Resistor of an RC filter and so as it changes it will not only affect the volume, but also the Filter's behavior!

Oh and about your idea of removing the input caps...don't. In tube circuits you gotta be careful about the Input voltage as it only works if its Below the Cathode voltage of the tube(hence the 150ohm at the cathode, It raises the cathode voltage to be above the Grid). If the input starts going over the cathode distortion will occur. with how every system has its own output tendency i wouldn't risk it. even a simple Film capacitor (or Good quality elektrolyte like the Nichicon muse BP) will prevent headache with future input devices!

Lastly...Very very nice choice in components. Its great to see proper components being used where it matters! i may not be an audiophile, but i can recognize good components and layout when i see it!! a nice change of pace of seeing cheapo components a lot of the time!

akristoffer (author)2013-12-03

Nice project. I'd like to build it with some adjustments. No battery, just plug it in. So a 12 volt adapter I guess, how many amps would it need? Also, output to speakers. What impedance might you suggest? I wouldn't be using an iPod, I'd be plugging her into my phone. Your project might not be ideally suited for my build, but it's something elegant like this or I'm gonna have to tear apart some computer speakers...which falls more than a bit short. I'd appreciate any suggestions. Again, awesome job. I'd like to put a quality tubeless amp up against something like this and decide for myself which sounds better.

JoeBeau (author)akristoffer2013-12-03

To make sure I answer everything I'm breaking this down:
1) The circuit is power hungry compared to normal solid state, but it will only consume as much power as it needs; it doesn't need a regulated supply. I would figure a 2A minimum supply should be fine. I would recommend the addition of RF filtering to the power supply if it doesn't have it already, as well as plenty of AC filtering.
2) I would avoid low-impedance speakers. I hooked this up to 16 ohm speakers and it was fine and went plenty loud. However, those speakers were also 98 db/w (decibel/watt) efficient. This amp is only about 2 to 3 watts max, so it would be best with high-efficiency speakers. Most modern speakers are not high efficiency, being around 87 db/w. Every 3 decibel increase is twice as loud, so efficiency is important. Many vintage speakers are high efficiency because they were designed to be used with lower power tube amps. My WIBAQ speakers featured on instructables are 95 db/w. With the introduction of semi-conductors, it became cheaper to achieve high volume output from speakers by pumping them with a lot of power than it would have been to just make them use power more efficiently. If you intend to use computer speakers as your speaker and use them for near-field listening, 2-3 watts is plenty.

akristoffer (author)JoeBeau2013-12-06

Just wanted to pop back in for a second. The link you provided for the circuit, now that I've checked it out, has info about using a power supply instead of a battery. He mentions 13v 1.8A regulated power supply stolen from a printer. I figured a printer power supply would be a good place to start. Also, in your parts list, you listed the tube as "2au7" and not "12au7". Confused me for a minute but I'm good. Any recommendation for filtering? I assemble, I certainly don't design anything this complicated. I'm sure the info is out there if I look for it. It wouldn't have crossed my mind if you didn't mention it. Thanks again.

JoeBeau (author)akristoffer2013-12-06

Thanks for the editing mistake. It's all set now.
If the original source mentions that for a power supply, then I would say go for it. After your question I did do a quick scan of the original article but must have missed the mention of the power supply.

By 'filtering' I mean adding extra AC filtering caps (Wired from DC+ to -). Adding an extra 1 to 5 thousand uF should suffice. This is just to add extra smoothing and make sure no noise from the power supply (since it is a switched supply for a printer) makes it through. Also, the extra capacitance acts as a 'power reservoir'. this prevents any transients, with their extra power draw, tripping the safety elements of the supply if the amp suddenly draws a lot more current. Also, adding a RF balun on the power cable going into the power supply is a good idea, though it most likely has one.

If you look at high end amps, they often have power filtering capacitance measured in farads. You can overdue it though, since the capacitors draw current while they are charging. Too much capacitance means too much of a draw on the power supply.

I will end this by saying that my suggestions qualify as 'over engineering' for something like this. It can't hurt if you have got the parts already though

ToXiCATOM (author)2013-11-22

plz,can u give us a picture when it is glowing?

terry999 (author)2013-11-02

Just wondering on the volume pot. On the original diyaudioprojects.com/ site, it indicates that pot should be 10K. Would using the 50K instead be a problem for the input impedance, or performance?
Would 10K be better, or is there a reason for the difference?

terry999 (author)terry9992013-11-03

Ok, I figured out that any range from 10K to 100K is probably fine, 50K being quite common. So, just need to adjust the wiper wiring per 'mpep' below.

llittlepage (author)2013-10-28

These aren't complaints. They're called "constructive criticisms". The author was kind enough to share his project and the readers are kind enough to share their knowledge and experiences. In fact, I think this kind of behavior is encouraged. I hope you don't take offense in what I was trying to say.

And by the way... Nice instructable JoeBeau!

JoeBeau (author)llittlepage2013-10-28

No offense taken. That's the point of having comments on an Instructable. It is important, however, to be able to filter out the unnecessary criticisms and extract the ones that posses useful information.

KROKKENOSTER (author)2013-10-28

Nifty circuit and WHO SAID THAT ELECTRON TUBES ARE DEAD?

fazgard (author)2013-10-26

Well documented - great build.

As an old tube guy, it's great to see them in current circuits!

mpep (author)2013-10-25

I hope it is a mistake in your drawing/schematic as the volume control is wired incorrectly.

JoeBeau (author)mpep2013-10-25

How so? It could be where the wiper is the 'output' to the circuit and one 'stationary pin' is 'input' and the other ground. But functionally it would be the same.

mpep (author)JoeBeau2013-10-26

If you have a 'input' on the wiper, and you adjust the volume, you may end up with the situation whereby the incoming audio is grounded. This should never occur.
Hope you can see why.

The wiper is, universally, the output to the amplifier. Have never seen any instance where it is not!

Just trying to help you out. Surprised no-one else has mentioned it.

sconner1 (author)2013-10-23

I recommend leaving filter caps on the input.
Here's why. Amplifier circuits amplify the input, even if the input has DC offset.
At worst, it can burn out the coils in your phones, at best, it causes clipping distortion at lower volume.
The filter caps will remove any offset and make sure the audio signal is pure AC.
Many designers even use coupling caps between stages of an amp design as extra protection. You can minimize tone loss by calculating a cap value that will put the rolloff frequency below the audible range, try 0.1uF. Also choose a poly or mylar cap and Not ceramic disc or electrolytic for this purpose. Electrolytics are better suited for smoothing power supply rails and delivering the power for bass response and transients.

mbelu (author)sconner12013-10-23

You're meaning coupling caps.

sconner1 (author)mbelu2013-10-25

Right, coupling caps.
The goal is to find/calculate the value that doesn't filter within the audible range.

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