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Too much sibilance in my newly built amp.

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Posted on July 24, 2016 at 01:54:01
DAK
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 I just finished a PP EL34 class A amp using a SE Schade feedback amp topology (the somewhat well known RH Universal amp by Alek Kitic). The amp sounds really good except for a tendency for pronounced f and s sounds on certain music. I have built 2 other RH Universal SE amps without this affect, so, i wonder if there maybe something else that may be causing the problem. Perhaps, the use of metal film resistors? I am using Vishay RNC90Y for the grid stoppers. The coupling caps are PIO. cheers, Dak

 

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RE: Too much sibilance in my newly built amp., posted on July 24, 2016 at 03:59:42
FenderLover
 I'd try carbon comp resistors as grid stoppers & see if that sibiliance relaxes.

 

If not the Carbon Comp grid stoppers for oscillation then..., posted on July 24, 2016 at 06:31:30
Chip647
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 try not using a zener for the screen source or put a small cap at the screen end of it. Nasty little things unless they are used as voltage references. There will be current flowing through this zener.

 

RE: Actually the power tubes are in UL mode, posted on July 24, 2016 at 10:37:26
DAK
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 I am using EICO HF 87 output trannys and i did not want to fuss with doing the added parts for the G2 supply. regards, Dak

 

RE: Could it be a break in issue?, posted on July 24, 2016 at 10:44:46
DAK
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 The Vishay were not cheap and since i mounted them very close to the socket I may not be able to salvage them after removal. Do you suppose they would "break in"?

 

Grid Stoppers MUST be Carbon Comp, posted on July 24, 2016 at 11:18:29
Triode_Kingdom
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 The grid stoppers MUST be carbon comp. I can't stress that too strongly. You may also need to cut open a sample to be sure the ones you buy are really carbon comp. Manufacturers have been cheating for at least the last 20 years; many "CC" resistors are actually spiral-wound carbon film. That entirely defeats their purpose for this use.

An effect sounding like excessive sibilance can also be caused by intermodulation distortion. Pentodes generate a lot of that, although it's not usually a problem in UL mode. Just be sure the external NFB loop is working as intended.

 

Vishay RNC90Y, posted on July 24, 2016 at 11:29:29
Triode_Kingdom
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 The image below is from the Vishay datasheets. It says "Conceptual Illustration," but the actual construction is likely to be similar. Note that the path length is much longer than in a carbon comp resistor with a solid body. This additional length creates inductance. Also, you can believe that their claim regarding cancellation of mutual inductance cannot be taken at face value where RF is involved. At higher frequencies, these resistors will not behave like a straight piece of wire.





 

RE: Grid Stoppers MUST be Carbon Comp, posted on July 24, 2016 at 11:42:33
DAK
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 I have built many versions of this circuit with different power tubes and have used different types of grid stopper resistors. And none of the amps really had this issue. The amps were single ended versions of this circuit. This circuit with Schade fb which goes from the output tube plate to the driver tube anode.

 

When you say sibilance,do you mean harshness or exaggerated notes like when the treble is turned up?, posted on July 24, 2016 at 16:29:35
Michael Samra
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 I have always felt that term doesn't belong in audio as I don't feel equates even tho so many people use the term.If you are talking about the musical notes straining,that might be considered sibilance I suppose.

Anyway,if this is a PP version of the amp we were talking about a few weeks back,you need to look at your square wave at 1khz and then 10khz just for purity.If it can't produce a 1khz sq wave without ringing,we will have to find out why.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: When you say sibilance,do you mean harshness or exaggerated notes like when the treble is turned up?, posted on July 24, 2016 at 17:19:08
DAK
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 Yes, the amp is the one, i think. PP class A amp with fixed bias.
The amp seems to exaggerate the f,s, and t sounds. With some recordings showing more of that tendency. But, i believe that some of that is less now that the amp has more hours on it.
The sibilance was most pronounced on "The Pentangle, Basket of Light". With Jacqui MacShee? vocals had a real obvious exageration on f's and s's.
Now that the amp has played for almost 20 hours it is not as bad. I have been playing it today for almost 8 hours and i played that Pentangle cd again. The track is called Lyke-Wake Dirge. It is mostly male and female vocals recorded live. I think the recording has exaggerated the esses in consonants. Some of the other tracks with the same vocalists are not as "hot" sounding.
Perhaps, i am being too picky, but other than this, the amp sounds very good indeed. cheers, Dak
PS, perhaps, it is a testament to that amp that the recording's deficiencies, has been laid bare so obviously.

 

RE: When you say sibilance,do you mean harshness or exaggerated notes like when the treble is turned up?, posted on July 24, 2016 at 17:44:24
Tre'
Industry Professional

Posts: 17293
Location: So. Cal.
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 J Gordan Holt "sibilance; A coloration that resembles or exaggerates the vocal s-sound."

I believe if we all adopt JGH's glossary we could communicate better.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Remove one leg of the Vishay, posted on July 24, 2016 at 17:53:37
FenderLover
 Insert a similar ohm Allen-Bradley carbon comp. See if you hear a difference. If so, remove the Vishay. If not re-attach the Vishay.

Not much current passes onto the signal grid (at least I hope not much) --- I doubt you'll see much break-in.

BTW... you aren't seeing any significant DC leakage across any of your coupling caps? This may also causes oddball sounds to ride on top of notes and voices.

 

Also..., posted on July 24, 2016 at 17:58:36
FenderLover
 How new are your caps and resistors in the power supply?

 

RE: When you say sibilance,do you mean harshness or exaggerated notes like when the treble is turned up?, posted on July 24, 2016 at 18:15:30
Michael Samra
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 I would use paper in oil coupling caps because when there is DC voltage on the cap,paper in oil caps maintain linenerity with frequency change.I still would look at a 1khz sq wave and see what the final product looks like as that can tell you a lot.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: When you say sibilance,do you mean harshness or exaggerated notes like when the treble is turned up?, posted on July 24, 2016 at 19:03:22
DAK
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 Hi Mikey, i am using PIO coupling caps. Do you think i should bail on the Vishays? regards, Dak

 

RE: Good idea, posted on July 24, 2016 at 19:05:47
DAK
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 I might try it but the space is pretty tight. I will play the amp and accumulate at least 100 hours before deciding. thanks for the help

 

Agree, posted on July 24, 2016 at 20:58:22
Triode_Kingdom
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 The strained sound Michael is discussing is probably better described as "stridence." That's very different from sibilance. Anyone who wants to know what sibilance sounds like need only find a pentode amp and remove the NFB. It will be immediately obvious what sound we are discussing here. :)





 

RE: When you say sibilance,do you mean harshness or exaggerated notes like when the treble is turned up?, posted on July 24, 2016 at 23:42:22
Michael Samra
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Posts: 36118
Location: saginaw michigan
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 i should bail on the Vishays? regards, Dak
If you are using them as grid to ground in high stages,they are great.I use them in preamps but if you using them as plate,cathode,or grid stoppers,they aren't the best bet in that scenario.There are many Vishays tho,I speak of the RN65 or any metal film.
Are you able to run a 1khz sq wave thru the amp to see what it looks like?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

Be interested in your results, posted on July 25, 2016 at 07:02:26
FenderLover
 I've never used anything, but a carbon comp resistors in grid stopper position.

 

RE: Be interested in your results, posted on July 25, 2016 at 07:08:57
lovetube
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 but most vintage CC resistor is out of specs by now. would they still be used ?

 

RE: Be interested in your results, posted on July 25, 2016 at 07:53:59
FenderLover
 I have a bunch of NOS A-B resistors. Measure before use.

 

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