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Output coupling cap and transformer mod...?

post #1 of 38
Thread Starter 

First off, thanks WALL-E for the detailed PM discussions, its been very educational.

 

I want to upgrade the output capacitance on my darkvoice 337 to improve bass response with my lower Z phones.  OEM cap and layout is shown below.

 

Ignoring the math and first order high-pass filter calculations for just a moment, subjectively the amp sounds surprisingly good for such under-valued capacitance.  Its just missing that last little sub-octave below~60Hz, well its probably missing more than that really.  Its NOT a bass shy amp at all, plenty of impact and clean dynamics... its just that bottom "BLOOM" area of the spectrum is lacking. The OEM capacitors are MKP, 400V, 51uf.

 

By my calcs I need about 250-300uf to really open up the TS-5998 tubes.

 

The problem I have is one of space.  If I remove the two chassis standoffs at the corner of each cap I will have a volume of about 85x77x51 mm to work in.  So I need to cram 250uf into that volume.

 

With MKP and other film caps Its like trying to cram a can of cambells chunky soup into my jeans... I dont think its going to work.

 

QUESTIONS:

-Do I really need a 400V cap?

-Plate voltage in the TS-5998 is 275V, is the cathode output voltage really up to 400V?   Was that value chosen by the manufatcturer more for design safety factor, low-ESR, sonic fidelity... or something else?

-How would I go about safely measuring cathode output voltage?  For obvious reasons I am a little hesitant to probe my fluke around inside a hot circuit.

-What are your thoughts on using low-ESR electrolytic caps for this application?  They are MUCH smaller.  I can very easily fit a 300-400uf - 400V electrolytic reservoir in this space, and room to spare for a smaller bypass cap of film or paper in oil.

 

 I  modded the chassis bottom cover with 5V cooling fans under the power resistors so heat really is not a concern, so long as the caps don't physically touch the resistors.

 

 

 

 


Edited by kramer5150 - 2/23/13 at 9:41am
post #2 of 38

Those are Solen caps BTW. but 400V was probably chosen for extra voltage headroom as you don't want these caps to fail... ever. 

post #3 of 38
Thread Starter 

OK thanks.

 

One nagging question I have then,  is why would they intentionally under-spec the capacitance to such severity?... planned obsolesence? ... IE plan for an "enhanced" LOW-Z optimized, higher $$$ version later down the road?  (which is eventually what they did)

post #4 of 38

Solen caps jump from 250 to 400V ratings. You *could* get by with a slightly lower voltage rating, but reasonable practice is for the output cap to be sized to handle AT LEAST B+ so nothing lower than 300V under any conditions. It might open up a few options in electrolytic caps, but mneh. 

 

I'm not a big fan of how OTL amps work with low impedance headphones, so I would lean away. 

 

If you want to do it anyways you should be able to find electrolytic caps in the size (physical dimensions, and electric capacity) and voltage ratings you need.

I am biased towards just a straight electrolytic OVER a bypassed electrolytic - I find the bypass cap leads to a certain smearing of the sound. 

 

If you don't want to upgrade the caps, but still want to do something - You may have good results with adding parafeed output transformers or autoformer (600:32ohm, 1K:8ohm, etc), and just running them off of the high impedance output jacks with the caps you have now. 

Hammond makes a 600:8 transformer off the shelf. There are a few places that will custom wind at those impedances - a bit more expensive, but much nicer. 

 

As another option in para-feed land, some DIYers have used toroidal transformers as parafeed transformers. They make a very fun alternative to "real" transformers, and work much better than you may think. 


Edited by nikongod - 2/21/13 at 1:03pm
post #5 of 38
Thread Starter 

OK thanks!!  great feedback gents.

post #6 of 38
ESR does not matter for output caps. Consistent ESR does.

Assuming there is no feedback in the circuit, the Zout from a 5998 cathode follower is about 70 ohms. For the purposes of calculating the 3db point of a coupling cap, you add this to the load impedance. That means your 50u caps give about a 30Hz 3db point into 32 ohms. It is definitely going to be a trick to get a larger film cap in there. If it were me, I might try to just parallel a second 50u Solen cap in there. Alternatively, you could add some series resistance between the cathode and the cap (100R or so). There is already an impedance mismatch -- a resistor won't make it much worse.
post #7 of 38
Thread Starter 

aah OK then that makes a LOT more sense, because I am DEFINITELY NOT hearing a 3db down point of 98Hz.  Theres substantial bass response well below that point with my Grados.

 

Factoring in the additional 70 ohms from the tubes, I only really need ~100uf to get a 3db down point of 15Hz.  That changes the playing field.

 

oh and thanks Nikon for the transformer tip too.


Edited by kramer5150 - 2/21/13 at 3:34pm
post #8 of 38
Looking at it again, 5998's are dual triodes. I don't know the circuit, but if they put the two sides in parallel, then the Zout is more like 35 Ohms. Or, it could be a different circuit. At any rate, you are probably correct that 3db is lower than you thought, but it may not be as low as I thought.
post #9 of 38
Thread Starter 

OK great thanks again... I think each tube is configured in parallel.

 

I found this low ESR Panasonic... how does this one look for this application?

 

http://media.digikey.com/pdf/Data%20Sheets/Panasonic%20Capacitors%20PDFs/EET-ED2W561EA.pdf

 

thanks ... again and again and again ;^)

post #10 of 38
Thread Starter 
Quote:
Originally Posted by nikongod View Post

Solen caps jump from 250 to 400V ratings. You *could* get by with a slightly lower voltage rating, but reasonable practice is for the output cap to be sized to handle AT LEAST B+ so nothing lower than 300V under any conditions. It might open up a few options in electrolytic caps, but mneh. 

 

I'm not a big fan of how OTL amps work with low impedance headphones, so I would lean away. 

 

If you want to do it anyways you should be able to find electrolytic caps in the size (physical dimensions, and electric capacity) and voltage ratings you need.

I am biased towards just a straight electrolytic OVER a bypassed electrolytic - I find the bypass cap leads to a certain smearing of the sound. 

 

If you don't want to upgrade the caps, but still want to do something - You may have good results with adding parafeed output transformers or autoformer (600:32ohm, 1K:8ohm, etc), and just running them off of the high impedance output jacks with the caps you have now. 

Hammond makes a 600:8 transformer off the shelf. There are a few places that will custom wind at those impedances - a bit more expensive, but much nicer. 

 

As another option in para-feed land, some DIYers have used toroidal transformers as parafeed transformers. They make a very fun alternative to "real" transformers, and work much better than you may think. 

So I found the Hammond 119DA, and it looks like it will almost get me there.  Can I just wire a pair of these in an enclosure with 1/4 TRS connectors in and out, and plug my low-Z cans into that?

 

http://www.hammondmfg.com/pdf/5c0055.pdf

 

With a 32- ohm secondary load, wouldn't that result in a primary load somewhere north of 2000 ohms?  Would that be too high of a primary load for my amp... and unfold other problems?

 

thanks!!

post #11 of 38
Quote:
Originally Posted by kramer5150 View Post

So I found the Hammond 119DA, and it looks like it will almost get me there.  Can I just wire a pair of these in an enclosure with 1/4 TRS connectors in and out, and plug my low-Z cans into that?

 

http://www.hammondmfg.com/pdf/5c0055.pdf

 

With a 32- ohm secondary load, wouldn't that result in a primary load somewhere north of 2000 ohms?  Would that be too high of a primary load for my amp... and unfold other problems?

 

thanks!!

 

I like the idea of just putting the transformers into their own little box, and have done the same when I did this for a Bottlehead CRACK. It lets you move them from amp to amp and saves the trouble of figuring out where to cram them in the bigger amp. 

 

There are a few ways to look at the weird primary/secondary impedance. 

 

To start with: The DV337 has a simple cathode follower output stage. Simple cathode followers don't require a load to work properly, so going from 600ohm to 2Kohm (or more) is not an issue. So don't worry here. 

 

There is a possible issue with the transformers. Because of how transformers are made transformers DO like to have a load. They are not as picky as certain speaker amps that can blow up if they are run without a speaker hooked up, but transformers often have a certain way they work best. 

 

There are 3 ways of looking at the problem of figuring out how they work best. In order from worst to best they are:

 

1: Forget it! Just act like its a non-issue and don't do anything! 32ohm speaker (or headphone) on an 8ohm transformer - no problem, act all casual and stuff... like nothing we do maters. This is BS, but it happens. 

 

2: Take the assumption that the MFR's recommendation for the secondary load is reasonably accurate and then calculate what resistor gives that load when placed in parallel with the headphones and just plug that in. Since that transformer has an 8ohm secondary we can assume that it wants an 8ohm load. 12||32~8ohms, nice and easy, and almost always better than #1. 

 

3: Measure with function generator and oscilloscope. Don't worry, I have not gone off the deep end into the madness of measurements first design BUT measuring can speed things up SO much and can save you a lot of frustration in trying to hear things that may not be there. Play a 10Khz square wave through the WHOLE system and measure what comes off the secondary on an oscilloscope. Try a bunch of loads on the secondary (I use a pot) and look for a nice square wave with no overshoot BUT also the steepest leading edge possible. 

 

So you know how I said that #1 was BS, sometimes it isn't! There are some transformers that don't really care what impedance loads they have on their secondary. Mysterious beasts... I am sure there is a trade off in their construction or performance but I don't know what it is. Measuring will tell you quite quickly how the transformers you have behave - which means that even when it doesn't matter if you measure or not, it matters. 

 

Secret option #4

Rough tune your gear using #2 or 3 and then adjust to taste by ear. 


Edited by nikongod - 2/22/13 at 8:40pm
post #12 of 38
Thread Starter 

OK thanks Nikon... I did a little google-foo, low and behold the mad ear+ (and other maple tree amps) uses the 119DA, after output capacitors.

 

Can you sanity check me on this... I don't see any signs of a parallel load resistor on the hammond output.  Unless its a TINY 1/8W tucked under there somewhere.  So it looks like the mother of all Grado synergy follows your scenario #1.  Is it possible the mW output on these amps is too small to "stress" the transformer?  How risky would it be to copy this and not use a parallel load resistor in my amp?  I am assuming that 12 ohm resistor is going to soak up some wattage, and I;d be better off without it if possible... is that correct?

 

thanks!!!

 

schematic...

http://www.veiset.net/mad/manual_EP.pdf

 

 

 


Edited by kramer5150 - 2/22/13 at 9:55pm
post #13 of 38
To clarify a bit -- a cathode follower needs a load. It is provided by the resistor between the cathode and ground in parallel with whatever is being driven -- the phones, the next stage, etc. If you remove your phones, then it is just the resistor. At a minimum for good performance you really want it to be at least 3x rp of the tube. The higher the better, which is why people put CCSes under cathode followers. It is also the reason that these big cathode follower amps have high distortion -- the load provided by the phones is too small. So, driving a transformer will likely yield better results than driving the phones directly.

When designing transformers, there are several tradeoffs. To overly simplify, for a given core size made from a given material (steel, or nickel, or whatever), each loop of wire can handle a certain amount of voltage (before saturation) and adds a certain amount of inductance. So, as you add more turns, the voltage handling increases as does the inductance. But so does the DCR and capacitance. So, if you want more power handling, you can increase the core, or increase the number of turns, with drawbacks to each strategy.

For the 119DA's, Hammond designed them for connecting 8 Ohm speakers to amplifiers that expect to see a 600 Ohm speaker. In the design process, in order to increase power handling, they had the option to make the core physically large (which is expensive) or add more wire turns, which is cheap. I'll let you guess which they did. This increased the inductance, which means that while they are rated at 600:8, the reality is that they are much higher impedance (the impedance is 2piFL where F is the frequency and L is the inductance). The benefit to us is that they can reflect a much higher load than 8 Ohms. So connecting your phones across the secondaries without any additional load is fine. You can add a resistor if you like but it should not be necessary.

Last, if you do use these, the 50u caps are larger than they need to be. With Grados, consider them to be 2400 Ohms and size the caps accordingly. Or don't. The 50u's will be fine. But you can shrink them if you like.
post #14 of 38
Thread Starter 

solid, thanks dsavisk, Nikon and all... great info there.

 

If I mount the transformers in a separate enclosure, they'd be grounded through the sleeve of a TRS plug + jack and additional wire length.  Am I likely to encounter ground loop issues?  I guess the only way to really know is just try it out... right?

 

Also I just noticed DV put parallel load resistors on the TRS jack.  Are these going to interfere with or disrupt anything, with the 600 (er ~2400) ohm primary coming after that resistance?  Should I remove those resistors all together?  Probably safer to just leave them as is...?

 

thanks


Edited by kramer5150 - 2/23/13 at 11:17am
post #15 of 38
Quote:
Originally Posted by kramer5150 View Post

If I mount the transformers in a separate enclosure, they'd be grounded through the sleeve of a TRS plug + jack and additional wire length.  Am I likely to encounter ground loop issues?  I guess the only way to really know is just try it out... right?

 

Also I just noticed DV put parallel load resistors on the TRS jack.  Are these going to interfere with or disrupt anything, with the 600 (er ~2400) ohm primary coming after that resistance?  Should I remove those resistors all together?  Probably safer to just leave them as is...?

 

 

This will not cause ground loop issues except in the most unusual of cases. 

Specifically, if everything is sitting on a grounded surface (desk or metal filing cabinet) and the chassis of the enclosures ground through that. 

The odds of this happening are really low. 

 

The load resistor on the TRS jack is there so you don't get annoying pops when you plug the headphones in. DO NOT REMOVE IT!

How many ohms is it? 

2Kohm or more is no problem. 

If it's less than 2Kohm replace it with 4.7Kohm. 

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