Darkvoice and Crack Mods

Let me be the first to start this thread.

I know there is much to discuss about Crack mods, as many of them they sell directly. I will start this thread to discuss different mods to this or any similar designed OTL cathode follower amp.

Mods I have done to my Darkvoice 336:

1. Disconnect the line output.



The 0.47uf series cap and the 2K resistor to ground cause some type of loading to the output. If you look at the Crack schematic:

[REDACTED]

You will see there is no line out, but it has a similar headphone output with a decoupling cap of 100uf in series and a 2.49K resistor to ground.

For the DV circut, the high pass cutoff frequency would be about 0.53 Hz.
For the Crack circuit, the HPF cutoff frequency would be about 0.64 Hz.

Both use the same design, but the Crack uses a higher value cap and a lower value resistor, but achieves the same result. But somehow the extra capacitance and resistance to ground on the DV moves up the HPF to 1.2Hz, but also somehow contributies to somehow loading the output. So for mine I disconnected the Line Out all togehter, but one could simply remove the connector to ground essentially taking that part of the circuit out of the equation.

This change simply lifted the "veil" on this amp. I compared it to a Crack initially and the DV was much darker. After removing the Line Out loading issue, they were close to the same given the same tubes. The Crack might have been slightly better given better components and design overall (better transformer, correct heater voltage, and less resistance on headphone out, even if it did have an electrolytic cap on the output).

2. LED cathode bias on the preamp tube

This is a fairly simple mod and also one I borrowed from the Crack schematic. I simply removed the 1K cathode biasing resistors on the 6SN7's and replaced with LED's just like the Crack. This allows for a much more steady biasing voltage.



This change made it even closer to the Crack. It retained much more detail and clarity and less sag. Not as big a change as lifting the Line Out, but much improved.

=================================

Other ideas for DV Mods:

1. Fix the high 7V heater rails.

It seems this is the biggest cause of the hum in the preamp tubes. Some 6SN7's are more sensive to high heather voltages and why different tubes as well as "burning in" or leaving it on can cause the hum to go away. One person fixed it by converting the AC heater rails to DC. This would work and would actually likely knock it down closer to 6.3V, but might cause some extra strain on the transformer as the specs on the transformer are unknown and may not have suitable amps for this section of the transformer.

2. Take out the LED on the heater rail and replace with a lamp.

There is a safer way to incorporate an LED on a heater rail, but this is a bad implementation. There are better configurations, but I would almost rather take it out the LED all together.

3. Drop the 220 ohm resistor in series with the plate of the 6SN7.

This will only increase the plate voltage and increase the headroom. There is already a 30K resistor in line there that will do just fine. Still not sure if taking this 220 would change the bias or operating point of the 6SN7.

There are other mods too, but most of those are replacing components (Alps pot, MKP caps, etc.).

Any other ideas?

 

Replace the 1k and 30k resistors with CCS loads. You can get these from K&K audio. Crack has its own mod called the speedball which is highly recommended.

BTW, you probably shouldn't post the exact page from the Bottlehead manual since its a copyright issue. You can however post your own schematic.

 

Right. Which is one of the Crack upgrades that I kind of mentioned that are on their own site. The Speedball is certainly an option, even for the DV the Speedball can be installed and the circuit draws little current, but there is certainly less physical space in the DV for those boards, but I imagine it is doable. I will check out K&K. Thanks.

Noted. I forgot that the manual is now password protected to only those who are buyers. The PDF manual though is available if you know how to Google search.

 

The circuit is in a lot of amps like the Millett Wheatfield HA-2, Apex Teton, Woo WA3 and Aren van Waarde on the old Head-Wize site. Here is a schematic of the van Waarde.

http://www.illuwatar.se/project_pages/6as7_headphone/images/amp_schematics.gif

I don't know what the power supply of the Dark Voice is but you can probably benefit a lot from squeezing in a choke somewhere.

 

Did you have a question or something to add @Gallic Dweller?

 

I've been reading various reviews from this site for some time but seeing this thread prompted me to join.

I decided to get a D/V 336 SE via Massdrop because I had never tried an OTL or any kind of equipment built using point-to-point and it used a very versatile driver tube type of which I have many, it was also a good price. Even better because it was sent 'in transit' to Germany before being sent on to France I did'nt have to pay any VTA or Customs inspection fee.

Of course the first thing I did was remove the base plate and take a look - nicely put together. Replaced the base plate and fired it up. Surprisingly from new and using the supplied Chinese tubes the sound was very good and straightaway I could hear the attraction of OTL amps.

I'm wondering how much of the dynamism of this amp is to do with the 250K pot? only a small amount of movement possible past 9, not surprising.

The single channel diagram you posted has 2 resistor values at variance with the amp I have - 6SN7 pin (1) yours 100K, mine reads and measures 1M brown/black/black/yellow, same tolerance 1% : your pre-amp resistor 2K, mine reads and measures 1M brown/black/green, same tolerance 5%.

If I can improve on the overall sound of the D/V then I want to keep the pre-amp part of the circuit.

Mods - your cathode idea is good - what value of LED did you use, was it HLMP 6000, is this value critical?

Re. the 250K pot - the Alps Blue isn't a good volume control but a the 'shunt mod' changes this, highly rec. check out www.world-designs.co.uk - FAQ-Upgrades and Tweaks. I use Z foils transforms the sound completely. Are you sure you need as a high a value as 100K? maybe 10 or 20K is enough.

If your going to use the D/V only as a h/amp here's a thought - use a selector switch instead fitted with high quality (Z foils, my choice) resistors. Just how many positions do you need with a h/amp?

Or why not try a DACT clone stepped att. , fitted with 1% SMDs, very highly thought of on diyaudio.

As for the o/put caps - space is a problem here. I have some Sonicap 10uF 200VDC but this would mean piggybacking one of them, not much of a problem for me as I will use the amp without the baseplate. However my favourite coupling cap by a mile is the cheap as chips Russian mil. spec. K73-16, they are as transparent as Z foil resistors.

The problem is that though I have 22uF ones they are only rated @ 63V. I know from previous work done by others that the o/put caps caps only see 70-75V. I'm sure the Russian military rated these caps very conservatively - dare I take a chance using 1 x K73-16 + 1 x Sonicap - what do others think?

If I find the pre-amp idea is not feasible then it will be easy to convert the output RCAs to another input source. I use vinyl as primary - Kenwood KD990 / AT 33EV, soon to be ART 9, Talk Electronics MC3 all discrete phono stage,all i/connects are own design, multi-conductor, high purity silver or silver/plated solid core air dialectric and a Project CDP.

The bog standard D/V sounds OK as is with the vinyl rig. With mods and changing all signal chassis wiring similar to my i/cs it should sound very good indeed. I use Oliver Heaviside's unchallenged findings on electrical conduction (late 19th century) as a basis for all signal conduction, which means that all the solid core point-to-point wire will be changed for quality copper either with FEP oversized tubing (air dialectric) or bare, protected with Pro-Gold against corrosion.

 

I never measured the pot. Crack uses 100K. I only guess they made a change to 250 K for finer control and that it will not get as loud.

This is very critical to the circuit. It sets the bias of the 6SN7 tube.

Not sure what you are asking. I used this one https://www.digikey.com/product-detail/en/lite-on-inc/LTL-4221N/160-1139-ND/214530

I think I did this based on the 2V of forward voltage. That is about the only spec that matters when you set an LED for cathode biasing on a preamp tube.

Look at the bottom of this page with some LED ideas:

http://www.valvewizard.co.uk/OtherStuff.html

Obviously an Alps, higher quality film, or stepped attenuator is always going to be an improvement.

I think the output caps were chosen because of they are film (although they are polyester), and quite large for their size, so there are some physicial limitations. I have seen some quite crazy cork sniffer caps on Crack amps, but that amp has a bit more room for such large value film caps.

63V is pretty low. I usually go with at least double the DC voltage they might see. Last thing you want to do is blow a cap. Although I do believe the voltage is distributed through all three caps. Measure the DC volts you see on that cap at the pin of the tube to ground with program material playing and across a load and that should tell you the voltage.

You are talking about a big change for extremely little change. I would focus on other areas. With changing the wiring and some of the other components, you are going to be limited by the input transformer and circuit design as well as the chassis in regards to space.

I still say the best areas to look in to making changes to the DV or Crack is setting a fixed bias on the pre-amp and amp tubes. It is not advertised, but you can install the Speedball upgrade on the DV 336. This is a huge upgrade. The LED upgrade on the DV 336 is already nice.

Removing the line output also is a big improvement. On other amps usually swtich this by having a headphone jack switch that disconnects the line out feed when headphones are inserted. If you want line out, I suggest doing this kind of mod.

And check your heater voltage. I know in the US, mine was like 7V, because I heard that the reason they are so cheap, they are using 100V/60Hz transformers for Japan that push the 6.3V secondary up to 7V. Not sure on EU models. If I had a Variac I could have easily fixed this by lowering my 125V power down to 100V, but I really should not have to and one of the reasons why I ultimately sold my DV. I don't mind doing a few mods, but I don't want to fix engineering mistakes that I should not have to.

So don't just throw some high dollar parts at it in hopes that it will improve the sound. Zfoils and SonicCaps are cool, but they do not make the amp.

 

rt - thanks for the response.

Re. the pot - far from a 'finer control' the useable range is minimal, why? It's been suggested on diyaudio that 10K would be fine. I don't know if you looked at the mod on World-Designs but shunting the pot will result in a minimum drop of 5.5/6dB - you can choose to drop a lot more gain by using a different value resistor. The main point is that the sound quality goes right up - the difference is not subtle. I know with my hybrid h/amp I removed the shunt and went back to the bog standard Alps Blue - the sound was really crappy. I have 20K and 50K unused Alps Blue pots which I can use. There is a forum on AA devoted to OTLs I shall ask the question there about pot or stepped att.s values.

The bias resistor - yes critical - why did your amp have a 100K and mine a 1M, doesn't make sense - the other 6SN7 resistors were the same - your 1K is a 1.1K in mine.

Output caps. - Others (Rock Grotto et al) have measured the voltages these caps see @ 70/75V most caps are tested to at least 1.5 times their rated value and the K73-16 are Russian mil.spec. so I'm sure they are tested at more than 1.5 times - this applies to mil. spec components everywhere - get this wrong and soldiers/sailors die. I may take a chance on these because they are so very good. I already have these 22uF caps and they are so cheap.

Signal wiring - only now are a few commercial companies waking up to Heaviside's seminal work on electrical conduction. Any piece of gear benefits from having signal wiring changed to suit this man's unchallenged findings - not expensive. Why is signal wiring always dressed around the sides of a chassis - because it looks neater, no other reason. When I change the wiring I always go in a straight line from input to pot / pot to o/put.

Your h/phone jack switch is a god idea.

Have you ever used Z foils? I have used Z foils in the signal path. Just by changing out 1 pair for anything else and you can hear the difference - Kiwames/Rikens/A/N tantalums et al - you can hear them all - the Z foils are like a piece of wire. I can only see a need to use 4 of them.

I intend to use Ohmite instead of Mills - not expensive. The 0.47uF K73-16s could'nt be cheaper or better if you want neutrality, if you want to add a little warmth pay serious money for Mundorf Silver/Gold, not me.

I only intend to use 14 or 16AWG copper UP-OCC for the signal bearing point-to-point wiring - 1 metre is about $7.

So the mods in total are not much, especially if the right bias control resistors are 100K, I already have these - vamos a ver.

As we are moving to a rented house for (I hope) about 2 years, the rooms are all too small to use my speakers I may well shell out for NIB Oppo PM2 - they don't come cheap.

 

I did the LED mod, yes it works, thanks.

 

I rtaylor76 - I know you plan to change the pot for a 100K Alps but why not just attenuate the existing pot? see www.goldpt.com/mods.html. Gives all the info on values of resistors needed for various levels of attenuation.

BTW - I de-activated the pre-amp out part of the circuit and again another good idea of yours.

Output caps - looking at rtaylors pic of the D/V circuit for the life of me I cannot understand why the PSU caps weren't placed further away from the o/put caps, there's plenty of room and the power resistors legs could easily be bent downwards as well. Both the h/amp and pre-amp resistors (behind the o/put caps) could be soldered underneath the PCB and the pre-amp caps (best changed for better ones) could have their legs bent downwards same as the power resistors - leaving plenty of room to fit big o/put caps.

For those using 300-600ohm cans and find that 30uF o/put caps work fine I found by accident (Google let me down) there is a seller on ebay.italia - ural spirit (100%) who has Russian mil. spec. K73-16 10uF/100V and 250V caps @ $2.50 per piece, shipping is cherap @ $6 + 30c per additional cap. The 100V caps are 49mm L x 20mm W. They may just fit longways and will easily fit if the PSU caps can be moved further back.

These are seriously good bi-polar caps and transform any piece of equipment they are used in, either amps or speakers. Ural Spirit will match pairs for free.

Next mod - change the signal wiring/input and R&L out from the pot, same as my i/connects. Then - 1M resistors from pins 1 & 4 removed and replaced with Z foil 100K.

 

Replaced the resistors on pin 1 & 4 with 100K Z foils - oh dear doesn't sound better, the zing has gone. Still can't understand why and the weirdest thing of all - I swapped the o/put tube from a Mullard 6080 for the GE 6080 and it sounds terrible, screeching, had to whip it out and replace with the Mullard. Neither of these results do I comprehend - bizarre.

I shall add a 1M resistor to an order for parts, can see no reason to change the other 6SN7 resistors.

I have to say that before the last resistor change I have never heard voices so 3 D before - Mark Knofler/Joni Mitchell/Paul Williams.

 

How I don't know why but I totally misread the value of the resistors on pins 1 & 4. Normally I would lift/desolder one leg of a resistor in a piece of gear to measure the value. However the Chinese who make the D/V still use the old method of establishing a 'mechanical' fix re-inforced with solder. So when I have changed a resistor in the D/V, I have simply cut the leg and soldered a new one onto the remaining wire/solder connection.

Imagine when I did what I should have done before soldering in any other value - measured the resistor I removed and found it was 1K ( the schematic shows 100K).

I have several 1K Z foils soldered into a hybrid amp I am rebuilding, so, removed 2 of these and soldered them into the D/V. The other change I have made after my previous post was to remove the Chinese 200 Ohm soldered to the PSU caps and replace with Mills.

Fired up the amp and here it gets bizarre, really bizarre - instead of the amp going loud around 9 o clock, it now only goes loud at 12.50/10 to 1, this I don't understand at all.

The very good news is that the Z foils have worked their magic again. Replacing the metal film 1K with Z foil 1K has really opened up the D/V, this is exactly what they did with my hybrid head and power amps. Ironically the first resistors after the Alps Blue were 1K as well.

I never thought I would say this but even after minor but important changes (the o/put cap change is yet to come) the sound I am now hearing is better than through my Heybrook Sextet Mk 1V speakers.

I know a lot on this forum don't like the Beyer sound because of their treble harshness and I did find the same with the DT 990 Premium cans but using Z foils in the signal path has removed this glare/harshness. Yes Z foils are expensive but worth every penny - they will hold their value precisely even @ 70C.

I am certain that changing our those first pins 1 & 4 metal film for Z foils will bring a big smile.

 

Can anyone help me out - I've been looking at various caps I could use instead of the Chinese polyester ones fitted. - 3 x 10uF per side. Decided against using my all time favourite Russian K73-16 as the kind of value I need (100uF) would (a) only be available as a multiple and the voltage is only 63V and that is taking a big risk.

So, I'm looking at 100uF per side, I'll stay with bipolar. I'll keep the PCB and use 1mm silver pins, so I can try different caps.

Here comes the ask - I want to try some Parts Express 4 x 100uF/100V non polarised @ $1.59 each - great price and bypass with 2.2uF 250V K73-16s. Then I looked at the cheapest option for shipping -to France - unbelievable $37. - and that was the cheapest. just plain crazy. Not only that, the cost of the 4 caps + shipping then put's me over the threshold to get hit for a 20% import duty.

I've got other bipolar caps to try for the D/V and for speaker x/overs hence the need for 4. All these caps need is a jiffy bag for postage. I don't know what the shipping cost is inside the USA but it can't be much, neither can cheapest postage (USPS) to France - please PM me if you can help.

 

@Gallic Dweller - have you looked at what's available at audiophonics.fr ?

 

So no one can do me this small favour - esta la vida.

Cspirou - I've looked there's nothing. Polyprops with the nec. values are hugely expensive. I may try Audyn ELKO 100uF/100V rough foil bipolars, 1 per channel and use either a K73-16 1600V /0.0047uF or a K73-16 250V/0.01uF as bypass.

The high voltage/small value K73 has been tried and rated as a bypass cap. There are some NOS ROE and others that I can try.

 

Hi to both Galic Dewller and Cspirou,
I’ve just got a DV no hum but pot needs changing and as I don’t need the pre I was going to disconnect it so I can use the spare phonos as feed thro for a tape.
My question is in my amp just behind the orange caps and in front of the PSU electro caps I can see the 0.47uF cap but next to it is a 1meg resistor not a 1k, is this the one to cut. A little in front and to one side is a pair of 10k’s. It looks like the 1meg is the one.
I can’t get to the underside of the PCB atm so was just going to cut the legs.

I was going to do basic mods as on here above remove pre, add Z Foil, and shunt pot. If I get any hum add the led.

Any help advise would be gratefully accepted

 

Pingu,
the existing pot is really easy to remove, replace with a 50K Alps Blue and shunt that with a Z foil. just cutting the legs is fine. I removed the PCB with o/put caps and I can say with authority - it's a bitch to replace as the Tx wiring is really, really short.

The o/put caps can be replaced with new not NOS 1 MIEC 220uF/100V caps per side from www.juastradio.com in Canada. It's impossible to get to the Chinese o/put cap legs without removing the PCB - put the ampm on it's side and destroy these caps any which way but observe that the legs are very puny. By keeping these legs intact you can solder new caps to these legs. The MIEC caps are 18mm x 39mm soldered diagonally they are a doddle to fit. Yes the 1 meg is the one.

 

Galic dweller have tried these people
They have 220Uf bipolar at reasonable price and postage is only about $7.50 to uk so I expect same to France.

https://www.justradios.com/resistors.html

 

Thanks I’ll just cut the legs on the 1m resistor to disconnect the pre and sort out the volume pot with a couple of extra z foils. Take it the WAD forum shunt is ok with only minimal attenuation10db or so.
Think I’ll leave the o/p caps alone for the mo. Maybe upgrade later when I got all else sorted out.
Good advise
Thanks again
Martin