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DarkVoice 336SE Circuit Overhaul

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Sep 3, 2022
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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.
Thank you @raindance - any idea where should I look out for this earthing connection? I've attached a couple of pictures of the unit... sorry I have a rather basic understanding of electronics principles ..
 

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raindance

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Your issue needs to be repaired before you think about mods. The schematic shows me that the input is not capacitively coupled and there's a bias path through the pot even if the 1M resistor goes bad. The other place to look is to verify the loading resistors are good on the output side, but even then, whatever mystery amp you're connecting it to would provide some sort of load.

I used to know a great tube amp guy in Jo'burg but he got murdered a few years back, sadly. See if you can track down Glen Turrell in Westville (Durban area) and see if he can suggest anyone to fix your amp.
 
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Your issue needs to be repaired before you think about mods. The schematic shows me that the input is not capacitively coupled and there's a bias path through the pot even if the 1M resistor goes bad. The other place to look is to verify the loading resistors are good on the output side, but even then, whatever mystery amp you're connecting it to would provide some sort of load.

I used to know a great tube amp guy in Jo'burg but he got murdered a few years back, sadly. See if you can track down Glen Turrell in Westville (Durban area) and see if he can suggest anyone to fix your amp.
Thank you I appreciate your feedback - I'll try find Glen on FB.. so is what you're saying that even if it is a bad resistor, it wouldn't be this type of issue? Using a Legacy LS200 as the power amp btw. Sorry to hear about the chap in jhb . Let me try Glen - thanks again
 

raindance

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Thank you I appreciate your feedback - I'll try find Glen on FB.. so is what you're saying that even if it is a bad resistor, it wouldn't be this type of issue? Using a Legacy LS200 as the power amp btw. Sorry to hear about the chap in jhb . Let me try Glen - thanks again
Glen is a well known musician in Durban. He may be elusive via Facebook, so try phoning if you can find a number.
 

SIY

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Your issue needs to be repaired before you think about mods. The schematic shows me that the input is not capacitively coupled and there's a bias path through the pot even if the 1M resistor goes bad. The other place to look is to verify the loading resistors are good on the output side, but even then, whatever mystery amp you're connecting it to would provide some sort of load.

I used to know a great tube amp guy in Jo'burg but he got murdered a few years back, sadly. See if you can track down Glen Turrell in Westville (Durban area) and see if he can suggest anyone to fix your amp.
There was a very experienced, smart, and super nice tube guy there named Johan Potgeiter who would be great to ask. Don’t know if he’s still around?
 

Stinius

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There was a very experienced, smart, and super nice tube guy there named Johan Potgeiter who would be great to ask. Don’t know if he’s still around?
Hi Stuart
I guess you mean Johan Potgieter ("Ampdog" in AVForums South Africa), as far as i know he passed away in june this summer.

Stein
 

SIY

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Hi Stuart
I guess you mean Johan Potgieter ("Ampdog" in AVForums South Africa), as far as i know he passed away in june this summer.

Stein
That’s a pity, he knew his stuff and was incredibly kind and generous. We had a nice correspondence for a while.
 

Nate29

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Hey @L0rdGwyn, I own a DV for awhile now and have had fun with it. Came across your work on modding this tube amp and I have a few question(s)

1) Seems like the most worth while mod is:
" 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."
** Seems like you have address the hum issue with this mod and more. If I do this mod, should I still consider the Fitz mod (parallel 220uF cap)?

2) Would you mind rating the complexity, $$$ investment, and overall improvement on SQ on the other other mods that you have listed (1-7)?
* I know this is a big ask, I just wanted to see how much further I want to invest in modding the DV. It does seem like a fun side project. Maybe this will also be useful for others noobies who's looking to mod their DV.

3) Are any of these mods (1-7) inter-dependent of each other? For ex: Can I do ONLY #3, because from the text it sounds like it's another mod to go along with #1.

Much appreciated,
Nate
 
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L0rdGwyn

L0rdGwyn

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Hey @Nate29 ,

Creating a virtual center tap and elevating the heater supply is worthwhile if you are having issues with noise / hum. You could try the center tap first then elevate the heaters if there is still a noise issue. I would try that first before the "Fitz mod". This mod is simply just adding a bypass cap to the input stage cathode resistor, which is left unbypassed in the original design. Bypassing with this cap will introduce it into the AC signal path, slightly increase the gain of the input stage, and slightly raise the distortion as the cathode will longer be degenerated which does introduce some negative feedback.

The other mods I wouldn't bother with, they are overly complex and expensive relative to what the DarkVoice is.
 

Nate29

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Thanks for the quick reply @L0rdGwyn !

Yep, was just trying to see if I want to add more things to my mousser order to save on shipping. But thanks for stopping me from going further down the rabbit hole.

I will at least play around with your center tap + elevated heaters mods first. Sounds like fun and I'm excited to try.

-Nate
 

BPLeader

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I have a similar problem with sound cutting out. DV will work fine for a few minutes, then sound in both channels fades away to silent. Simultaneously, the glow from the rear tube goes out. I've replaced both tubes with same results. Any thoughts? Unit is over a year old.
 

SIY

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I have a similar problem with sound cutting out. DV will work fine for a few minutes, then sound in both channels fades away to silent. Simultaneously, the glow from the rear tube goes out. I've replaced both tubes with same results. Any thoughts? Unit is over a year old.
Sounds like the heater power supply is the issue. Is there a regulator like an LM317 which is going into a foldback state?
 
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L0rdGwyn

L0rdGwyn

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Sounds like the heater power supply is the issue. Is there a regulator like an LM317 which is going into a foldback state?

As far as I can remember, it is a passive AC supply with one leg tied to ground, no center tap.
 

lashto

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Just glancing at the circuit, the parts used look to be comparable, it is just built out on a PCB as opposed to point to point wiring.
Guess that is how they managed to make it smaller.

The XDuoo specs (0.1% THD-N and 110dB SNR) are much better than ASR measured for Darkvoice but not sure if one can trust those manufacturer numbers. And some youtube 'reviewers' say that it also sounds better (whatever that means).
Would be interesting to know if there was anything in the xDuoo circuit to support such claims. Does it have better power regulation? Better parts? Did they address the grounding/humming issues? Does it have more/less NFB...

P.S.
this review has extra internal pics (see the Build and Internals sections), maybe that helps
 
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