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DarkVoice 336SE Circuit Overhaul

L0rdGwyn

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The DarkVoice 336SE is one of the more popular entry-level tube headphone amplifiers, it was reviewed on this site to much criticism. I was introduced to a group of individuals on Head-Fi who were making component changes / modifications to their own circuits. I thought it would be a fun project to do my own "modifications", a nice Head-Fier sent me his spare stock DarkVoice to do with what I pleased.

Here is a photo of the stock circuit.

DV336se  .jpg

A quick description: it uses a passive CRCRC-CRC supply. The 6SN7 input tube is loaded with a plate resistor and cathode biased, the cathode resistor is unbypassed. The 6SN7 plate is then direct-coupled to the 6AS7/6080 grid. The 6AS7/6080 cathode follower is biased with a 1K 25W wirewound resistor. The output is capacitor coupled with 30uF per channel, giving a f-3dB point of 17Hz with 300ohm headphones and an output impedance of roughly 75ohms. As such, using anything less than 300ohm headphones leads to significant LF rolloff.

I thought the circuit would benefit from a regulated power supply, an active load on the 6SN7, as well as other various "fixes" based on what I discovered inside.

Here is a quick summary of the changes I made:

1) Removed CRCRC-CRC filter from the power supply - replaced with single 330uF resevoir cap and Maida style HV regulator. This allowed an increase in the B+ to 180VDC from ~140-150VDC in the stock design with much better PSRR.

2) Removed plate resistor load from 6SN7 input tube - replaced with DN2540 - J310 "gyrator" load. This fixes the plate voltage of the 6SN7 and allows the tube to find its plate current based on the bias. The reference voltage is generated via a LND150 cascode CCS across a resistor to ground. I designed the PCBs to mount on the existing standoffs from the final CRC of the passive supply.

3) Removed 6080 1K wirewound cathode resistors - replaced with Vishay 1.2K non-inductive wirewound resistors, adjusting the bias point to account for the higher B+ and maintain the bias current of the original design.

4) Removed 6SN7 1K cathode resistors - replaced with KOA Spear 300ohm carbon film resistors, again to adjust the bias.

5) Created a virtual center tap via 2x 100ohm resistors on the heater supply. Elevated the heater supply 50VDC above ground. Added twisted heater leads (previously straight, solid core 18AWG wire). This cured the well-known hum issues of the amplifier, as the original designer simply grounded one leg of the heaters.

6) Increased the output capacitance from 30uF per channel to 66uF per channel. This gives a f-3dB point of 20Hz with 120ohm headphones.

7) Power LED is run from the heater supply. As such, rerouted the power LED ground to the heater supply center tap to float it on the elevated supply, adjusted current limiting resistors for the halved voltage due to the newly balanced supply.

Here are the new tube bias points:
B+ 180VDC
6SN7: 80Va / 4.5mA / 1.4Vk
6080: 75Va / 85mA / 105Vk

There are a few other things to do (startup protection diodes on 6AS7/6080 grid, replace volume pot, replace the power switch per the request of the original owner), but overall the circuit is more or less complete.

IMAG3067-2-min.jpg

The sound is drastically improved and 6SN7 tubes that were previously unusable due to almighty 60Hz hum are now completely silent.

IMAG3069-2.jpg

Not really a DarkVoice anymore, but it sure sounds good :) might build an improved version of this circuit in my own chassis without the size limitations.
 

dfuller

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Very cool stuff indeed. I'd love to see some measurements now.
1) Removed CRCRC-CRC filter from the power supply - replaced with single 330uF resevoir cap and Maida style HV regulator. This allowed an increase in the B+ to 180VDC from ~140-150VDC in the stock design with much better PSRR.
This part with the HV regulation is very interesting to me, and it's something I'd like to mess with in a guitar amp (mostly for the preamp, plain old CLC and CRC does okay for the power amp).
2) Removed plate resistor load from 6SN7 input tube - replaced with DN2540 - J310 "gyrator" load. This fixes the plate voltage of the 6SN7 and allowed the tube to find its plate current based on the bias. The reference voltage is generated via a LND150 cascode CCS across a resistor to ground. I designed the PCBs to mount on the existing standoffs from the final CRC of the passive supply.
This isn't something I've ever really messed with - what's the benefit of it?
5) Created a virtual center tap via 2x 100ohm resistors on the heater supply. Elevated the heater supply 50VDC above ground. Added twisted heater leads (previously straight, solid core 18AWG wire). This cured the well-known hum issues of the amplifier, as the original designer simply grounded one leg of the heaters.
What's interesting is you can achieve quiet amps with straight heaters, though I've only ever really seen it in Soldano guitar amps.
 

SIY

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This part with the HV regulation is very interesting to me, and it's something I'd like to mess with in a guitar amp (mostly for the preamp, plain old CLC and CRC does okay for the power amp).

This isn't something I've ever really messed with - what's the benefit of it?

What's interesting is you can achieve quiet amps with straight heaters, though I've only ever really seen it in Soldano guitar amps.

HV regulation is often a net negative in a guitar amp for exactly the reason that it's a net positive in a hifi amp- in the former, you actually want the sag as part of the "sound."

Active loads are pretty common. The big proponent of the gyrator type is Anatoliy Lisovskiy (Wavebourn Labs). A lot of different active plate loading schemes are discussed in Morgan Jones's and Merlin Blencoe's books.
 

dfuller

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HV regulation is often a net negative in a guitar amp for exactly the reason that it's a net positive in a hifi amp- in the former, you actually want the sag as part of the "sound."
Yeah, I'm only thinking the preamp would make sense in a guitar amp, just to reduce noise. Power amp in my experience better to leave it unregulated. Preamp clipping isn't really from voltage sag, more from going out of the linear part of the curves of the tubes whether from lots of signal or from some rather clever biasing tricks (the Marshall/Soldano "cold clipper" for example).
 
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L0rdGwyn

L0rdGwyn

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The gyrator load presents a high AC impedance load to the input tube, flattening the load line and reducing H2. It also has very high power supply rejection. Another proponent of the gyrator load is Ale Moglia.

I will try to post some measurements over the next few days, they will not impress anyone but are pretty typical for a circuit of this topology. I can tell you that THD at 1mW into 300ohms is roughly 0.14% all H2. FR is flat to 20kHz and down around 0.8dB at 20Hz into 300ohms.
 

Mossshine

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Would you mind showing thr mod #5 and what exactly you did, please? I would like to try it on my dark voice too.
 
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L0rdGwyn

L0rdGwyn

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Would you mind showing thr mod #5 and what exactly you did, please? I would like to try it on my dark voice too.

I am going to post some instructions on Head-Fi for others what want to make this change, will link it here when I do.
 

crm

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What is the merit of exchanging the heater wiring?
 
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The DarkVoice 336SE is one of the more popular entry-level tube headphone amplifiers, it was reviewed on this site to much criticism. I was introduced to a group of individuals on Head-Fi who were making component changes / modifications to their own circuits. I thought it would be a fun project to do my own "modifications", a nice Head-Fier sent me his spare stock DarkVoice to do with what I pleased.

Here is a photo of the stock circuit.

View attachment 112517

A quick description: it uses a passive CRCRC-CRC supply. The 6SN7 input tube is loaded with a plate resistor and cathode biased, the cathode resistor is unbypassed. The 6SN7 plate is then direct-coupled to the 6AS7/6080 grid. The 6AS7/6080 cathode follower is biased with a 1K 25W wirewound resistor. The output is capacitor coupled with 30uF per channel, giving a f-3dB point of 17Hz with 300ohm headphones and an output impedance of roughly 75ohms. As such, using anything less than 300ohm headphones leads to significant LF rolloff.

I thought the circuit would benefit from a regulated power supply, an active load on the 6SN7, as well as other various "fixes" based on what I discovered inside.

Here is a quick summary of the changes I made:

1) Removed CRCRC-CRC filter from the power supply - replaced with single 330uF resevoir cap and Maida style HV regulator. This allowed an increase in the B+ to 180VDC from ~140-150VDC in the stock design with much better PSRR.

2) Removed plate resistor load from 6SN7 input tube - replaced with DN2540 - J310 "gyrator" load. This fixes the plate voltage of the 6SN7 and allows the tube to find its plate current based on the bias. The reference voltage is generated via a LND150 cascode CCS across a resistor to ground. I designed the PCBs to mount on the existing standoffs from the final CRC of the passive supply.

3) Removed 6080 1K wirewound cathode resistors - replaced with Vishay 1.2K non-inductive wirewound resistors, adjusting the bias point to account for the higher B+ and maintain the bias current of the original design.

4) Removed 6SN7 1K cathode resistors - replaced with KOA Spear 300ohm carbon film resistors, again to adjust the bias.

5) Created a virtual center tap via 2x 100ohm resistors on the heater supply. Elevated the heater supply 50VDC above ground. Added twisted heater leads (previously straight, solid core 18AWG wire). This cured the well-known hum issues of the amplifier, as the original designer simply grounded one leg of the heaters.

6) Increased the output capacitance from 30uF per channel to 66uF per channel. This gives a f-3dB point of 20Hz with 120ohm headphones.

7) Power LED is run from the heater supply. As such, rerouted the power LED ground to the heater supply center tap to float it on the elevated supply, adjusted current limiting resistors for the halved voltage due to the newly balanced supply.

Here are the new tube bias points:
B+ 180VDC
6SN7: 80Va / 4.5mA / 1.4Vk
6080: 75Va / 85mA / 105Vk

There are a few other things to do (startup protection diodes on 6AS7/6080 grid, replace volume pot, replace the power switch per the request of the original owner), but overall the circuit is more or less complete.

View attachment 112525

The sound is drastically improved and 6SN7 tubes that were previously unusable due to almighty 60Hz hum are now completely silent.

View attachment 112526

Not really a DarkVoice anymore, but it sure sounds good :) might build an improved version of this circuit in my own chassis without the size limitations.
Hi there. I'm quite a new audiophile so please excuse any ignorance here ;) - using my darkvoice to pre-amp a legacy high current amplifier to magnepans. I love the sound I'm not having issues with humm or noise etc.. but one channel keeps fading away and coming back again aferr some minutes. You obviously know this circuit by now... the funny thing is that previously one just had to touch the body of the darkvoice and it would make a tiny static spark and come back to life again - SO bizarre! But now that doesn't happen any longer.. Sometimes stays off indefinitely, but seldomly on both channels anymore... it's rather frustrating... do you think this is this a tube or maybe capacitor issue? Is your upgrade available to purchase and post to South Africa?
 
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L0rdGwyn

L0rdGwyn

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Hi there. I'm quite a new audiophile so please excuse any ignorance here ;) - using my darkvoice to pre-amp a legacy high current amplifier to magnepans. I love the sound I'm not having issues with humm or noise etc.. but one channel keeps fading away and coming back again aferr some minutes. You obviously know this circuit by now... the funny thing is that previously one just had to touch the body of the darkvoice and it would make a tiny static spark and come back to life again - SO bizarre! But now that doesn't happen any longer.. Sometimes stays off indefinitely, but seldomly on both channels anymore... it's rather frustrating... do you think this is this a tube or maybe capacitor issue? Is your upgrade available to purchase and post to South Africa?

Hmm that's quite an unusual bug, are you within the return window? Honestly I would try to return it if you can, unless you have a tech nearby who could take a look at it, not easy to do over the internet. And I'm sorry, this was a one off "for fun" thing to do, so not available as an upgrade. If you did want an upgrade of sorts, you could look into getting a Bottlehead Crack + Speedball which uses active loads on the input and output stages, which is a big plus IMO.
 
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Hmm that's quite an unusual bug, are you within the return window? Honestly I would try to return it if you can, unless you have a tech nearby who could take a look at it, not easy to do over the internet. And I'm sorry, this was a one off "for fun" thing to do, so not available as an upgrade. If you did want an upgrade of sorts, you could look into getting a Bottlehead Crack + Speedball which uses active loads on the input and output stages, which is a
 
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Thank you @L0rdGwyn. Unfortunately it was purchased 2nd hand , I'm keen to make it work (stubbornly so haha).
I've replaced the 6sn7 with ecc33 and it is even better wow! I'm really happy with thevredult in my setup... but it's still fading off to one channel and then coming back again and so on, so its not that tube thats the issue... how I understand it , the bigger 6as7 cannot be responsible for only 1 channel..? What about the big red resistors - do you think one of them could be the culprit?
 

SIY

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Thank you @L0rdGwyn. Unfortunately it was purchased 2nd hand , I'm keen to make it work (stubbornly so haha).
I've replaced the 6sn7 with ecc33 and it is even better wow! I'm really happy with thevredult in my setup... but it's still fading off to one channel and then coming back again and so on, so its not that tube thats the issue... how I understand it , the bigger 6as7 cannot be responsible for only 1 channel..? What about the big red resistors - do you think one of them could be the culprit?
Do you have access to freeze spray? That may help pinpoint the bad component or connection. (Do NOT spray it on tubes!)
 
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Thank you.
It's possible to get branded versions but rather expensive due to import..
I've managed to find 99% Isopropyl Spray, is it the same thing?
 

SIY

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Thank you.
It's possible to get branded versions but rather expensive due to import..
I've managed to find 99% Isopropyl Spray, is it the same thing?
Unlikely. I'm not sure where you are, but if in Europe, I have some contacts I can ask.
 

raindance

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Thank you @L0rdGwyn. Unfortunately it was purchased 2nd hand , I'm keen to make it work (stubbornly so haha).
I've replaced the 6sn7 with ecc33 and it is even better wow! I'm really happy with thevredult in my setup... but it's still fading off to one channel and then coming back again and so on, so its not that tube thats the issue... how I understand it , the bigger 6as7 cannot be responsible for only 1 channel..? What about the big red resistors - do you think one of them could be the culprit?
Is the Darkvoice amp grounded? Sounds like it isn't.

These fading issues are usually due to a missing bias path in a small signal capacitively coupled preamp stage. The ground could be that path.
 
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