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A lo-cost HT-supply true transformer-coupled tube headphone amplifier

post #1 of 108
Thread Starter 

I designed this:-

 

 

 

Bits came yesterday. This is just the trial fit for the parts.

 

It says lo-cost in the thread title, but that's comparative. The parts cost is at least twice what you can build a very good solid-state amplifier for, $220 - $250 including chassis.

 

I haven't finalised the design, although I've got a pretty good idea where I'm going with it. When I do I'll have a more accurate costing.

 

Here's an early simulation, although I've changed a few things since then...

 

 

 

That's the nice thing about simulations, you get the opportunity to do revisions and tweak the performance. I've since decided that maybe I'll do without the global negative feedback, I can get a milliwatt into 16 ohms (110dB SPL in my Klipsch Image S3's) with about 0.2% THD (almost all benign 2nd. harmonic) with no GNFB. The amplifier is intended to offer about 300mW and drive > 166mA or +/- 9V, although with increased distortion. Exactly what can be done in terms of power and distortion is yet to be determined. I don't think anybody will be arguing that RMAA results are insufficiently accurate though.

 

Nominal output impedance is switchable, 4-32 ohms.

 

Obviously it's single-ended, class A 

 

As drawn the tubes are Russian 6N6P, $7 the pair. These are said to sound good when you get them over 8mA quiescent, both stages run 10mA. 6SN7's, which are widely held to have low distortion, could be substituted. 

 

The output transformers are Hammond general-purpose 125SEB, rated 5W and 45mA DC bias, 100Hz - 15kHz, 1dB. They're considerably underrun at the power and DC bias stated, and the frequency response should be considerably better than nominal, they should easily achieve 20kHz and will probably extend down to 20Hz with 1dB flatness.

 

The PSU uses a silicon bridge rectifier to achieve ~280V with a capacitor-input CLC filter, I originally drew in active regulation, but the choke makes for a much simpler implementation. Tube rectification is an option at the cost of some HT voltage, but for the moment I've got a relay drawn in to delay the appearance of HT.

 

Unfortunately I incorrectly dimensioned the choke in my haste so a more suitable one won't get here until about Wednesday, along with some other components which I'm still missing.

 

w

post #2 of 108

Subscribed. 

post #3 of 108

Thanks for your design and making the DIY forum interesting.  Looking forward to see this being built with your goals in mind and also with "spare no expense" upgrades which hopefully will follow by people who like to improve things.  I would like to build one along with the puppy DAC this winter.  What name are you going to call it?  Challenger, Wiki HP1000 or...
 

post #4 of 108

Fantastic work. I love your design process, objective and incremental improvement via simulation and testing. This is real engineering folks.

post #5 of 108

Always good to see a new tube amp design, particularly one that attempts performance, simplicity, and low cost all in the same package. Not enough of this type of project around here.

 

Having built a number of similar projects here's my $.02:

 

1) I'd change out the 6N6P for a 6EW7. The first section of the 6EW7 is similar to a 6SN7, The second is a power section with a u of about 5 and an rp of about 1k. This will be a much better match with your transformer primary than a 6N6P. There will still be room for a few dB of global feedback, which will probably help your output transformers.

 

2) Substitute an LED for the cathode resistor/bypass cap on the first stage. Think about a CCS as the plate load. With the IXYS10M90 high voltage current regulators, a CCS becomes trivial.

 

3) I like the CCS in the cathode of the second stage. Set the CCS to elevate the second stage cathode to the point where you can direct couple the second stage grid to the plate of your first stage, and thus eliminate the coupling cap, which is always a weak point in any amplifier.

 

4) While your design models well and your physical layout looks nice and tight, my guess is that that your magnetic components will interact with each other and you'll have hum problems. Unfortunately the only cure is often physical separation of components.  Sometimes a choke actually creates the problem. You may want to return to the idea of some sort of simple regulation.

 

5)  Good luck and keep us posted!

post #6 of 108
Thread Starter 

OK, thanks for the support and expressions of interest, particularly FrankCooter for the detailed and useful advice, I'm a bit out of touch with tube designs, and I'll take all that aboard. I'll probably proceed with the design as I have mooted it for the moment, but it is my intention to iterate it at least once. I've already had some doubts about the room available with this chassis. I think I'll put the choke on the outside of the end plate at right angles to the field of the toroid. It's not the best aesthetically, but there's the aesthetics of hum to consider. The 6EW7 looks good, do you know of a spice model? I've found a model rated 'very good' for the 6EM7.

 

I'm still mulling over what to call it.

 

Parts have been shipped and I should have everything necessary to complete a working amplifier tomorrow or Wednesday.

 

w

post #7 of 108
Thread Starter 

After dozens of simulations, this is what we're going with for the first try:-

 

 

This should live with 2V in without clipping. The transformer model is basic, I get 0.15% THD with 1mW into 16 ohms across the 4 ohm output, ~2.5% THD with 9V across 600 ohms for 2V in using the 32 ohm tap, with this limited accuracy simulation.

 

w

post #8 of 108

Since you're not using a tube rectifier you can use a very large cap after the SS bridge and lower the value of your choke to around 1H as most of the ripple will be taken care of by the big-ass cap, thereby eliminating any magnetic coupling of hum from the choke to surrounding circuitry.. The choke will do a nice job of filtering out all the HF nasties from the SS bridge and help to sweeten up the sound. The following cap can be a smaller value high quality polypropylene (not exotic, just run of the mill) and you will get very pure DC out of this filter. Then you can use a simple regulator after this filter if you wish.

 

I can't read your schematic, it won't go big enough so the digits are not pixelated on my small screen so I have no idea what values you're using for your caps and chokes but this is a method that I've used and it works.

post #9 of 108
Quote:
Originally Posted by FrankCooter View Post

1)
2)
3)
4)

Everything Frank said is absolutely correct. Listen to him.

Quote:
Originally Posted by wakibaki View Post

I get 0.15% THD with 1mW into 16 ohms across the 4 ohm output, ~2.5% THD with 9V across 600 ohms for 2V in using the 32 ohm tap

That's a little high, but the important thing is the shape of the distortion, not the aggregate. So long as harmonics are decreasing, it should sound fine.

Quote:
Originally Posted by elliottstudio View Post

Since you're not using a tube rectifier you can use a very large cap after the SS bridge and lower the value of your choke to around 1H .... The following cap can be a smaller value ...

This would be the recipe for making the power supply ring. PS caps should be in ascending size, not descending.

post #10 of 108

Yes, but if followed with a regulator or a large bank of caps located right at the supply of each plate you're covered. I was thinking in terms of the power supply feeding into a circuit with adequate bypassing at each plate. I forgot to mention that.

post #11 of 108

wow...this is awesome. Thank you guys for doing this out in the open.

post #12 of 108
Thread Starter 

You can find a .jpg generated from a 600 dpi .bmp here:- http://dl.dropbox.com/u/107775480/challenge_choke_1.jpg

 

It's slightly different from the schematic shown, but I'm always thinking of improvements, get rid of a component here or there...

 

w

post #13 of 108

What does the Relay do? B+ delay? Its completely unnecessary in this circuit. There is no shunt regulator waiting to explode. No truly unlimited current tubes that will get screwed up if B+ is applied before the heaters are hot. Will it send an annoying "pop" through the headphones when it turns on? Save some space, money, and a headache. 

 

Why do you spend so much on the heater supply when you put so little into B+? 

So I know the choke costs some serious money in a budget build, and it is a VERY nice addition, but it wont be enough by its self. Put the regulators on B+. Even if they are just series pass transistors with zener references - or the more complicated version in your original schematic from the other thread.

Use AC heaters biased above ground. Noise levels will be very significantly lower, cost should be close enough to the same once all things are considered.  

 

(edit - added next morning) 

AC heaters does not account for hopelessly microphonic Russian tubes. It may be prudent to stay with DC heaters - CRC should be quite adequate though. 

(end edit)

 

Nit pickery: the various connection points in the schematic dont all line up. The transformer has AC2 on BOTH sides of the HV secondary - the relay shows a connection to AC1. The heaters for U1 go from one V-reg to the other (LT1 to LT2) - not to ground (LT1 - LT3). 

 

Quote:
Originally Posted by elliottstudio View Post

Yes, but if followed with a regulator or a large bank of caps located right at the supply of each plate you're covered. 

 

If followed by a large bank of caps what you said a few posts up of "choke followed by a smaller cap" is nothing like what you are describing now. 

 

If followed by a regulator:

Do regulators work better with cleaner supplies or "dirtier"? Cleaner of course. Why subject the regulator to the nasty HF of a ringing power supply when you could feed it a simple LF ripple with very little HF to speak of for basically the same cost... 

 

The big cap straight off of the rectifier is still not a great idea with SS diodes. Its always nice to keep the peak currents low. 20uf is more than enough for all but the scary-largest of tube amps, but nobody believes it. Since this is a budget build I think it will be a nice place to save some money. If it were a "money is no object" amp I would use the same size part, just better quality. 


Edited by nikongod - 10/5/12 at 7:50am
post #14 of 108
Quote

If followed by a large bank of caps what you said a few posts up of "choke followed by a smaller cap" is nothing like what you are describing now.

Yes, you're right. I need to say what I mean and mean what I say.

post #15 of 108
Thread Starter 
Quote:
Originally Posted by nikongod View Post

What does the Relay do? B+ delay? Its completely unnecessary in this circuit. There is no shunt regulator waiting to explode. No truly unlimited current tubes that will get screwed up if B+ is applied before the heaters are hot. Will it send an annoying "pop" through the headphones when it turns on? Save some space, money, and a headache. 

 

(edit - added next morning) 

AC heaters does not account for hopelessly microphonic Russian tubes. It may be prudent to stay with DC heaters - CRC should be quite adequate though. 

(end edit)

 

Nit pickery: the various connection points in the schematic dont all line up. The transformer has AC2 on BOTH sides of the HV secondary - the relay shows a connection to AC1. The heaters for U1 go from one V-reg to the other (LT1 to LT2) - not to ground (LT1 - LT3). 

 

20uf is more than enough for all but the scary-largest of tube amps, but nobody believes it.

 

OK, you sure about that with the relay? I got the impression from the Morgan Jones book that a B+ delay was a good idea with a silicon PSU due to cathode stripping, but I can do with the simplification. It was easier not to take the risk. I wasn't joking when I said I haven't done this for 40 years.

 

The tubes are what I had. I'll look at different tubes when I see some progress. I need to lose 3+V though, 1.5A means a 10W resistor I haven't got lying around.

 

As regards the nit-pickery, I've redrawn this circuit a lot, the wire labels keep deleting themselves when I redraw the wires. I'm not dependent on the schematic, I'm not making a PCB. I wouldn't want anybody else trying to follow it though, so I'll post alterations soon.

 

I thought single-ended had poor PSRR, so I've got a lot of capacitance in there, I wish I'd known, these caps are huge and it's crowded already.

 

I haven't done a lot on it. I've picked up a chest infection. I had surgery and chemo for lung cancer a couple years back and I had radio iodine for hyperthyroidism about 6 weeks ago, I don't throw things off like I used to and I feel like sh1t so I've taken some prescription codein and settled down to wait it out.

 

Thanks for the help nikongod.

 

 

 

 

 

 

w

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