Issues Using Darkvoice 336i as a Preamp

[Capital]

I did a search where one other person was using the Darkvoice 336i as a preamp but the thread really didn't get into any details on what I'm experiencing.

After doing a lot of reading here I finally bit the bullet and purchased a Darkvoice 336i and a pair of AKG K701s. I had really been enjoying the Darkvoice with the headphones but opened up a whole other side to this device when I began using it as a preamp.

My system is fairly basic; on the source side I'm using a Technics/Panasonic 110 disc changer as a transport feeding a California Audio Labs Sigma II tube DAC (this uses a Mullard 12AX7). I am using two Adcom GFA-545 amplifiers that are vertically bi-amped to Dahlquist DQ-8s with Scanspeak and Vifa drivers and lately I've been using Paradigm Studio 20s.

Previously I was using an old Rotel 960ax preamp and the Darkvoice opened up my system to such a point that I was considering the possibility that the Rotel hadn't been functioning correctly. Bass was now full, the highs are nice a smooth with the Rotel being tinny in comparison. It's really amazing the difference this piece has made.

I am however having a couple issues. Both of which are heard either between tracks or while I'm changing a disk.

First, it seems that the Darkvoice has no kind of isolation (I believe this is the right word). Meaning, if the volume knob is slightly tapped an audible "thump" is heard through the speakers as if one was tapping on a microphone. I noticed this just isn't an issue with the volume knob but tapping anywhere on the chassis has the same kind of result.

Secondly, every now and then I start hearing what I can only describe as the cross between a hum and a ring. It's definitely not the tubes as I've had this noise initially and it's since gone away after some burn in time. However, when I give the volume knob a firm tap the noise is instantly gone.

Any ideas on what could be going on?

 

[Sarchi]

Quote:

Originally Posted by Capital /img/forum/go_quote.gif However, when I give the volume knob a firm tap the noise is instantly gone.

It could be the volume pot itself, or possibly a resistor nearby. If it were me, I'd open the amp up and gently poke at the nearby resistors and caps (and the pot connections) with a pencil, with the volume turned up. When you come to the one that buzzes, resolder the leads on it. But that's just me.

 

[Capital]

Thank you, I'll definitely give that a try and report back.

 

[Capital]

Sorry I forgot one more question. Does anyone know if there are schematics or a parts list available for the 336i? I'm most likely going to upgrade the pot and wanted to know what other parts may be worth upgrading.

 

[LoweArt]

I never had the problem you've reported when I had the Darkvoice 336i and I later removed the linear pot for the correct logarithmic pot ( ALPS ) which fixed the problem where I previously could only use the volume control between 7 to 10 o'clock position.

From my experience, the hum/ring ( especially when you tap on the any part of the amp's case or associated parts ) that you're getting may be a microphonics problem caused by faulty driver tube. I've had and heard of similar issues with other valve amps where this has happened and once I traced down the tube that was responsible and replaced it, all gone ! Try changing the front 6SN7 tube first for another one and see if this issue goes away.

Hope you're able to get it sorted.

 

[Jo6Pak]

LoweArt-

Can you advise us as to the part number of that ALPS pot and best place to get one? THANKS!

 

[Capital]

Quote:

Originally Posted by Jo6Pak /img/forum/go_quote.gif LoweArt- Can you advise us as to the part number of that ALPS pot and best place to get one? THANKS!

I was just going to ask that, looks as if there are several to choose from....

Mouser Electronics - Electronic Component Distributor

 

[LoweArt]

Hey Jo6Pak,

I got my ALPS Blue Velvet 100k X 2 logarithmic pot from a great chap named Brian Cherry who I have dealt with on several previous occasions in relation to getting parts related to valve amps. He runs a business called DIY Hi Fi Supply in Hong Kong, see - Diy HiFi Supply as well as having a shop in eBay, see user name diy_hifi_supply.

Now, getting the ALPS pot from Brian is the easy thing as he's great with fast dispatch/shipping. Once you receive the new volume pot, you have to decide how you are going to mount it inside the Darkvoice 336i as the existing volume pot is soldered onto a small PCB which your new ALPS pot won't fit back on to as it's physcially bigger than the existing one. I have heard other Head-fier Darkvoice 336i owners remove the old one completely ( PCB and all ) and just hardwire ( point to point ) solder the new ALPS pot in.

I looked into that, but considered it would be neater to replicate what was there, so I made myself a new PCB, soldered the new ALPS pot onto that and then re-soldered the connections of the Darkvoice onto the PCB.

see the old and the new ->

Also my previous thread regarding this mod ->
http://www.head-fi.org/forums/f5/vol...e-336i-236934/

Hope this helps !

 

[LoweArt]

Hi Capital,

I suggest you contact Brian Cherry as per my previous post to Jo6Pak and ask him to supply an ALPS Blue Velvet 100k X 2 logarithmic volume pot. He'll know and supply the correct one for you ! This volume pot will not only correct the problem with not enough volume variation through the dial, but the feel when you turn the volume dial will be much smoother too !

Hope this and the previous post helps.

 

[SoFlaChris]

Have you tried new tubes? Something sounds microphonic.

Quote:

Originally Posted by Capital /img/forum/go_quote.gif First, it seems that the Darkvoice has no kind of isolation (I believe this is the right word). Meaning, if the volume knob is slightly tapped an audible "thump" is heard through the speakers as if one was tapping on a microphone. I noticed this just isn't an issue with the volume knob but tapping anywhere on the chassis has the same kind of result. Secondly, every now and then I start hearing what I can only describe as the cross between a hum and a ring. It's definitely not the tubes as I've had this noise initially and it's since gone away after some burn in time. However, when I give the volume knob a firm tap the noise is instantly gone. Any ideas on what could be going on?

 

[LoweArt]

Hi Capital,

I just recalled going back to earlier in the year when there was a lot of BUZZING about the Darkvoice 336i there was mention of a HUM issue with some 336's and Fitz was "the man" who came up with a fix for it.

If none of the other suggestions work or resolve your hum/ring issue you might get some leads from the following ->

http://www.head-fi.org/forums/f5/dar...oltage-180333/

Cheers

 

[Fitz]

Quote:

Originally Posted by Capital /img/forum/go_quote.gif Meaning, if the volume knob is slightly tapped an audible "thump" is heard through the speakers as if one was tapping on a microphone.

These noises are, unsurprisingly enough, called "microphonics," where the tube is acting like a microphone for any physical vibrations. There isn't a whole lot that can be done about this besides using different tubes.


Quote:

Originally Posted by Capital /img/forum/go_quote.gif Secondly, every now and then I start hearing what I can only describe as the cross between a hum and a ring. It's definitely not the tubes as I've had this noise initially and it's since gone away after some burn in time. However, when I give the volume knob a firm tap the noise is instantly gone.

Sounds like a faulty or dirty connection somewhere, assuming it does this with more than one set of tubes. The stock volume pot and tube sockets are both rubbish, so they're both worth investigating. Next time it does it try rotating the volume knob to see if that clears it, and also try gently wiggling the tubes in their socket. At one point I had an old 6AS7 with some grooves dug into the pins from their previous home, which really fought against the tube sockets in the 336 and had a couple pins barely making any solid contact.


Quote:

Originally Posted by Capital /img/forum/go_quote.gif Sorry I forgot one more question. Does anyone know if there are schematics or a parts list available for the 336i? I'm most likely going to upgrade the pot and wanted to know what other parts may be worth upgrading.

I can probably provide you with my own schematics for the 336 if I can find where I saved them, but it doesn't have the specific values for the preamp outputs of the newer ones.

 

[Capital]

Thanks to everyone for the responses. I did some looking into microphonics, very interesting stuff. I'll be hitting up the for sale section for a new front tube and may as well do the rear tube while I'm at it.

LoweArt, thank you for the information on the DIY HiFi site, very cool stuff! I'll be in touch with him for an Alps Blue Velvet.

One more modification I stumbled across that I may just have to give a try while I have the unit open for the new Alps is the addition of .22mf capacitors in the output stage as described in KT88's thread here... http://www.head-fi.org/forums/f5/dar...rrived-202486/

Madisound.com has some Mundorf capacitors that look like they may work quite well.... Madisound Shopping Cart

On a side note, would replacing other existing capacitors in the output stage yield any benefits? It looks like the DarkVoice 336i has 6 10uF capacitors.

 

[Sarchi]

Quote:

Originally Posted by Capital /img/forum/go_quote.gif the addition of .22mf capacitors in the output stage as described in KT88's thread here... http://www.head-fi.org/forums/f5/dar...rrived-202486/ Madisound.com has some Mundorf capacitors that look like they may work quite well.... Madisound Shopping Cart

I can highly recommend the Jupiter beeswax caps, provided they aren't going to be exposed to excessive heat. These caps are a bit costlier than the Mundorfs, they do sound phenomenal though. I can't imagine many circuits where they wouldn't make a noticeable impact.

I'm guessing the 10uF caps are probably for PS filtering. Are they electrolytics?

 

[LoweArt]

Hi Capital,

You will find plenty of 6SN7 tubes for sale and at reasonable prices on eBay too. I don't know of your personal preference in regards to which brand, but I'm sure you'll easily find what you need/want.

There's always room for improvement as far as upgrading the caps to higher
grade ones, I believe FITZ has done some amazing upgrades in the past with his 336 but if memory serves the internal parts layout of the 336 verses the later 336i is a little different. The 10uf caps you're referring to in the 336i are
soldered onto a PCB where room is certainly at a premium as the caps are already "stacked" right next to each other, so be careful of what or how you are to fit in alternative types.

Have fun on what ever you decide on doing.