I am a building a pair of monoblocks amplifiers based on my two channel amp. The output transformer is ultra-linear PP. I have redesigned the circuit such that the power stage has four KT88 tubes (two pair connected in parallel - see attached schematic). The coupling capacitors are RTX Multicaps .22 uF 400V. My current design employs two capacitor, one for each side of the power stage (see schematic). My question is this: Would it be better to use four coupling capacitors (one four each power tube), or should I leave it like it is? Also, I have read a lot on coupling capacitors and it seems there is a raekl lack of consensus on the best. The RTX rate well but so do the Russian Surplus Military caps. I'm sure opinions vary widely but I would like to know what the group thinks. Thanks for your help.
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Four coupling capacitors would ruin your bias. Capacitors block DC and allow AC. With 0 volts on the control grids of the two extra power tubes, your project would cook. If the schematic you posted is what you plan on building, you should be OK. Make sure that your power tubes are closely matched and that the two triode tubes are matched. Actually, I would match every resistor in there to 1%. If you need help setting the bias and crossover point, I can help.
dicranurus; your schematic appears to be an output stage that is going to be running pushpull parallel with fixed bias. If that is the case, you will want a separate coupling capacitor to each output tube. By changing to separate coupling caps, you will be able to individually set bias for each output tube to the desired set point. Thus, having the capability of balancing both sides of output transformer for current when proper bias established.
Mickeystan
Mickeystan
In this case a single capacitor for each phase will work OK. Some hardcore audiophiles, and designers as well, think that even the most well matched coupling capacitors tend to smear the signal when more then one per phase is used. But this is debatable since I have never seen or heard any real proof of this. The circuit as shown will require well matched tubes. Should you choose to go with adjustable negative bias voltage for each tube (my personal preference), then four separate capacitors would be needed.
Now the reason for the lack of agreement as to the "best" capacitor is because there is no best! It's all subjective to ones hearing and taste. Too many other things affect what you hear from your system. Output transformers, speakers, crossovers, many other circuit components as well as signal source all play a part.
There is one general rule of thumb that you can use as a guide for capacitor choice. And this concerns the dielectric used in the capacitor. Paper (in oil) dielectrics are comparatively slow and so give a mellower sound with less emphasis on high frequencies. Faster dielectrics, like Teflon™ and polypropylene, pass higher frequencies better and produce a brighter sound. The type you finally end up using will depend on your ears and all the above factors. And whatever you do, stay away from the exorbitant priced boutique hyped-up audiophool brands. They have nothing to offer over a lesser priced quality part.
Now the reason for the lack of agreement as to the "best" capacitor is because there is no best! It's all subjective to ones hearing and taste. Too many other things affect what you hear from your system. Output transformers, speakers, crossovers, many other circuit components as well as signal source all play a part.
There is one general rule of thumb that you can use as a guide for capacitor choice. And this concerns the dielectric used in the capacitor. Paper (in oil) dielectrics are comparatively slow and so give a mellower sound with less emphasis on high frequencies. Faster dielectrics, like Teflon™ and polypropylene, pass higher frequencies better and produce a brighter sound. The type you finally end up using will depend on your ears and all the above factors. And whatever you do, stay away from the exorbitant priced boutique hyped-up audiophool brands. They have nothing to offer over a lesser priced quality part.
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my rule of thumb for coupling caps, use the smallest value you can get away with....
i do not subscribe to capacitor brands but by technology, that said, polypropylene is good enough for me...Spargue 715, Wima MKP10's are okey.....
i do not subscribe to capacitor brands but by technology, that said, polypropylene is good enough for me...Spargue 715, Wima MKP10's are okey.....
Are there separate bias controls for points C, and D? If there are then you need the separate coupling caps. Split up the four tubes so that the bias controls do not interact with each other. If there are only two bias controls then leave it as is otherwise you have bias for the extra two tubes. But you will need to match the tubes on each side.
The schematic looks like part of an Antique Sound Labs amp.
The schematic looks like part of an Antique Sound Labs amp.
Thanks a million for all the responses. The design is a fixed bias with separate controls so I will modify the design to accommodate separate capacitors for each phase.
Also I'll do a bit of thinking on the couplers I use.
Also I'll do a bit of thinking on the couplers I use.
The design is based on a Passion PAK-A11 kit manufactured by Rotac out of Quebec, Canada. I very much enjoyed this amp but it is a little under powered for my new speakers so I'm attempting this modification of the design, hoping to get nearly the same quality with more power (and having fun doing it).
I have already made my PCBs but it will be easy enough to modify them to accommodate the additional caps.
I have already made my PCBs but it will be easy enough to modify them to accommodate the additional caps.
I am a little puzzled by the great debate that takes place over capacitors--I wonder if anyone could enlighten me? In the long path from the microphones in the recording studio to the output from the home CD player, the audio signal will presumably have passed through quite a few capacitors, and they will almost certainly not have been any special russian oil-filled capacitors or anything like that; just standard off-the-shelf components. Why would it matter particularly whether the last two or three in the long audio path--the ones in the home stereo amplifier--were of some special kind or not? It is surely not the case that they will "put right all the damage" that was done to the audio signal on its way into the home stereo amplifier when it went through standard components in all the rest of the long journey?
So why would it make any big difference what kind the final few capacitors in the chain are?
Chris
Half of those here would say that it makes no difference. The other half know better from experience.
Try comparing a few different types of the same value, for example as coupling capacitors wired in boxes with in/out jacks
between your preamp and power amp, for yourself and see. Try comparing mylar with polypropolyene to start with.
Listen to one pair one day (or week), and another pair the next day (or week). Go back and forth until you can easily
recognize the differences. And no, you don't need to do blind tests to do this.
Well, I suppose I must be in the half who can't hear the difference. (And without blind tests, I wouldn't know whether differences I might think I was hearing were real or imagined.) Would you say, then, that irreparable damage is being done to the audio quality by the production studio if they are not using the right kind of capacitors in their processing and recording equipment?
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Anything in life can be done to varying degrees of refinement. Doug Sax went to much greater extremes than just the types of capacitors
when making direct to disc LPs, for example. I'll bet that you can hear differences in capacitors easily outside of a feedback loop. Give it a try.
Don't let some of these guys intimidate you.
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Well, I'd be interested to give it a try. So I should compare mylar with polypropylene, you would say? I'm not optimistic I'd be able to hear any difference, since I can't even hear the difference between my tube amplifiers and a standard type of solid-state amp. I suppose some would say my hearing must be deficient. At least I can try also measuring the electrical characteristics of the mylar versus polypropylene, since, unlike my ears, that would be something I would trust.
Mylars have considerably more distortion, that's why I suggested to start with comparing them to a very good dielectric like polypropylene.
Do you have speakers with a very flat impedance curve? Usually tube amps will have audible bumps in the response at the impedance peaks
of the speaker due to the tube amplifier's moderate output impedance.
Yes you are correct that all musical recordings have been corrupted in some way by the electronics used to make them. And there is little we can do about that except to try and limit what our systems do to it.
For many years I used to think capacitors were just capacitors and were all basically the same. We used to call them condensers. Then maybe some thirty plus years ago I read an article in Audio Magazine by Walter Jung and Richard Marsh about picking capacitors and their effect on audio frequencies. Some time after that I decided to modify my home built Ampex 351 recorder by replacing the paper dielectric capacitors I originally used with more modern (better?) plastic dielectric types. And so I did.
Wow, what a difference! The sound was awful. The highs were way too aggressive just from changing three or four capacitors (with the same value) in the signal path. Playback of my standard calibration tape verified this. I had to make serious playback and record EQ adjustments to restore normal sounding operation. For me this was a major learning experience about capacitor action and audio. What made it so startling was that there were frequency tuned circuits that were affected. Had there not been the results would have been more subtle but still audible.
Rayma I like you idea if inserting different dielectric capacitors in the signal path for listening tests. Although a single cap may be too subtle for some ears or systems. I would suggest comparing paper to polypropylene since Mylar™ (polyethylene) and polypropylene are closer to each other. For anyone with good ears it should only take one good track of vocal or small well recorded orchestra to hear it. But not poorly recorded rock and roll.
Picking Capacitors in PDF with graphics.
http://waltjung.org/PDFs/Picking_Capacitors_1.pdf
http://www.elpee.info/Documenten/PickingCapacitors2.pdf
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Chris, I think the more important issue is the proportion of the signal that appears across the cap. If the 1/wRC is a decade lower than the lowest frequency of interest, it's essentially zero.
So it still gets down to the basics- good tight windings, low leakage. Nothing mysterious or subtle.
So it still gets down to the basics- good tight windings, low leakage. Nothing mysterious or subtle.
You are right to question this.
A capacitor must be specified for the duty it needs to perform.
Selecting the wrong capacitor will sometimes result in poor performance.
Equally selecting the correct capacitor will usually result in good performance.
There seems to be much confusion between the two main duties of AUDIO capacitors: coupling and filtering.
These are quite different. They generally require different types of capacitors to match the duty they are exposed to.
There lies the clue:
select a capacitor that makes a good filter when filtering is required.
select a capacitor that passes the wanted signal when coupling is required.
Many that have got these mixed up end up guessing at what to do/try next.
Guessing rarely allows one to arrive at a conclusion that is applicable to anything beyond what was tested.
This isn't about audiophile vs standard caps,
You can hear a difference even between orange drop 715 and 716 types construction is different. One has steel leads and the other has copper.
Now irrelevant of the leads...steel etc because you might prefer the 715 over the 716..however its is there a difference.
You can hear a difference with Robert Hovland caps and something like an LCR standard cap. You can even hear a difference between the LCR audio grade and an LCR standard cap..NB I prefer the standard cap.
Try listening to Silmic 2 Vs cerafine electrolytics..in the cathode bypass function..then listen to a cap used in a SMPS..
Listen to the amount of bass or treble present..its a good start!
It doesn't even matter if you prefer a standard cap to an audio grade cap..the point is you have a preference...so there would have to be a difference.
Regards
M. Gregg
You can hear a difference even between orange drop 715 and 716 types construction is different. One has steel leads and the other has copper.
Now irrelevant of the leads...steel etc because you might prefer the 715 over the 716..however its is there a difference.
You can hear a difference with Robert Hovland caps and something like an LCR standard cap. You can even hear a difference between the LCR audio grade and an LCR standard cap..NB I prefer the standard cap.
Try listening to Silmic 2 Vs cerafine electrolytics..in the cathode bypass function..then listen to a cap used in a SMPS..
Listen to the amount of bass or treble present..its a good start!
It doesn't even matter if you prefer a standard cap to an audio grade cap..the point is you have a preference...so there would have to be a difference.
Regards
M. Gregg
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