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Tube coupling caps reduced from 2.7uF to 1.5uF

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Posted on July 16, 2024 at 21:32:33
Mendel
Audiophile

Posts: 1224
Location: GTA
Joined: January 17, 2009
 I recently had my 20 year old Rogue Audio Stereo 90 in the shop for repair and decided to have some modifications done to the amp. The amp began life as a Stereo 88, was converted by Rogue to a Stereo 90 about 15 years ago and modified about 10 years ago by a local tech (Dan Santoni RIP) At that time the tube coupling caps were changed to Mundorf Supreme at the original value of 2.7uF.
The current modification included changing these coupling caps to Mundorf Supreme SilverOil caps bypassed by Dynacap .01uF and bypassing the main power supply caps with Multicap PPMFX .068uF.
I got the amp back and was pleased with what I heard. Very clean and detailed sounding, a little light on bass and a little thinner in tone, but I figured the caps need breakin. When I got the invoice, I was very surprised to see that the Mundorf caps used for replacement were 1.5uF, not the 2.7uF caps that were in there. This is the value Rogue uses for these caps.
When I asked why this had been done, I was told that lowering the capacitance would "speed up the sound" of the amp, and that 2.7uF was too much. When I asked about bass response, they said the amp was tested flat to 10Hz and that normal coupling caps were .47uF or lower and that 1.5uF was more than enough for the whole audio spectrum. And finally I was told that the 1.5uF Mundorf Supreme SilverOil were the largest caps of that type that would fit.
So I picked up the phone and called Rogue. Unfortunately Mark O'brien is in Europe , but I spoke to a gentleman who also spoke to one of the techs at Rogue. Both agreed that the change would not harm the amp. They basically said to break in the caps and see if I liked the change. Both agreed that the Mundorf Supreme SilverOil were an excellent choice to use as tube coupling caps. Neither one could offer an explanation as to why Mark had chosen such a high value for these coupling caps (there must be a reason, bigger caps cost more don't they?).
Just wondering what people think about all this.......

 

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RE: Tube coupling caps reduced from 2.7uF to 1.5uF, posted on July 17, 2024 at 07:02:49
Tre'
Industry Professional

Posts: 17618
Location: So. Cal.
Joined: February 9, 2002
 What is the value of the grid resistors that the caps feed into?

For fun let's just say they are 100k.
2.7uf into 100k makes a high pass filter with a -3db point of .59 Hz
1.5uf into 100k makes a high pass filter with a -3db point of 1 Hz

In this example there would be no audible difference caused by the capacitance value change.

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

 

RE: Tube coupling caps reduced from 2.7uF to 1.5uF, posted on July 17, 2024 at 08:41:03
Mendel
Audiophile

Posts: 1224
Location: GTA
Joined: January 17, 2009
 Hi Tre'
Thanks for the response.
I can't answer your question about the grid resistors. The tech that did the cap change says that the amp is flat to 10Hz. I'm sure that he noted the value of the resistors.
The company that did the mods is not some tech working in his basement, rather it is a large parts supplier and repair depo, run by a guy who used to manufacture a well known line of amplifiers and preamps. So they would have a full set of test equipment and should know what they are doing.

 

So why are you stressing it.?, posted on July 17, 2024 at 09:40:04
ABliss
Audiophile

Posts: 1546
Joined: March 16, 2001
Contributor
  Since: August 3, 2002
 The silver/oil caps are probably emphasizing highs, so the bass doesn't sound a pronounced right now, but give it time, they will settle in and you will adjust to the sound.

 

RE: So why are you stressing it.?, posted on July 17, 2024 at 10:55:56
Mendel
Audiophile

Posts: 1224
Location: GTA
Joined: January 17, 2009
 Not stressing it. Just never heard of a 2.7uF cap being replaced by one with almost 50% less capacitance at 1.5uF. Didn't know you could do that.
And I appreciate the reassurance from those in the know. Time to break in those caps...-...-

 

You also added new bypass caps, posted on July 17, 2024 at 19:36:37
Jon L
Audiophile

Posts: 6168
Joined: April 6, 2000
 which will *tend* to lead to "faster" sound.
After complete break-in, if you still feel like "little light on bass and a little thinner in tone," I would try taking out all the bypass caps (both to coupling cap and PS caps) and listen. If still not there, I personally then would add back some small paper-in-oil bypass caps to the coupling cap..

 

Mundorf S/O caps, posted on July 17, 2024 at 19:46:36
madisonears
Audiophile

Posts: 1593
Location: midwest
Joined: September 6, 2006
 I have found Mundorf caps in general, and especially the Silver/Oil caps, to tilt toward high frequencies and be a bit lean in the bass.

I wonder why the tech thought it necessary to bypass them, possibly compromising signal integrity. They are very nice caps (at least in the high and mid frequencies) by themselves, and I don't expect they would benefit from bypassing.

The value is probably ok, and perhaps the low end will flesh out a little with break-in, but the basic nature of those caps is, in my opinion, always going to sizzle the highs and starve the bass.
berate is 8 and benign is 9

 

A few more hours of listening...-...-, posted on July 18, 2024 at 11:19:20
Mendel
Audiophile

Posts: 1224
Location: GTA
Joined: January 17, 2009
 Not sure if it's the burn in or my getting used to the sound, but everything is sounding very good. Much more transparent and faster sounding than before, seems to be less noise (lower noise floor). The tone seems to have filled out and the bass is there, tighter and less rounded than before.
Will be adding powered subwoofers to the system soon (getting 2 new 20 amp lines installed in August, one will be for my Acoustats and the two subs) so low bass response of the amp will be irrelevant anyway.

 

RE: So why are you stressing it.?, posted on July 22, 2024 at 14:28:40
Lew
Audiophile

Posts: 10965
Location: Bethesda, Maryland
Joined: December 11, 2000
 The coupling capacitor value in conjunction with the input impedance of the following stage together form a low pass filter, as Tre' pointed out. He showed that on the assumption that the grid resistor of the following stage is at least 100K ohms, which is reasonable, the low frequency cut-off is around 1Hz, even with the 1.5uF coupling capacitor value. This is much more than adequate to assure a flat response down to 10Hz. Even if the grid resistor is 50kHz, you are still in good shape. What I think may account for the slightly shrill sound quality is the presence of the .01uF Dynamicap in parallel with the Mundorf. I personally never bypass coupling capacitors because the bypass capacitor almost inevitably adds a hi-fi-ish sound that has a negative effect the sense of realism I seek. If you know how to do it, you might try disconnecting that bypass capacitor to see if you like the sound better. It's not needed, that's for sure.

I see that Madisonears essentially wrote the same thing, regarding the possible negative effect of the bypass capacitor in a coupling application. I agree with him.

 

RE: So why are you stressing it.?, posted on July 22, 2024 at 18:57:58
Mendel
Audiophile

Posts: 1224
Location: GTA
Joined: January 17, 2009
 Thank you Lew for the response.
I personally never understood this "bypass" thing myself. The only reasons I did it is because in their upgrade package for my amp Rogue bypasses these cap, and my local tech seemed to be in favour of doing it. This seems to be a contentious issue, some liking to bypass, others not.
That said, I am listening as I type this, the amp sounds superb. Far better than I remember it being before. Not sure if it's all the new caps breaking in or my ears getting used to the sound or a combo of both but I am happy so I'm not changing anything for now.
Easy to remove the bypass caps down the road or even substitute the Mundorfs for something else. My main concern was reducing the cap from 2.7uF to 1.5uF. Since that seems to be a nonissue, it opens up more options down the road. May even be able to fit a 1.5uF film cap in that spot now.

 

RE: So why are you stressing it.?, posted on July 23, 2024 at 06:20:14
Lew
Audiophile

Posts: 10965
Location: Bethesda, Maryland
Joined: December 11, 2000
 Isn't the Mundorf SG already a film and foil cap as opposed to metallized film?

 

No they are metallized...-., posted on July 23, 2024 at 07:40:02
Mendel
Audiophile

Posts: 1224
Location: GTA
Joined: January 17, 2009
 Could never fit a film 2.7uF on the board. But maybe a 1.5uF film might fit.

 

RE: So why are you stressing it.?, posted on July 23, 2024 at 08:55:01
Lew
Audiophile

Posts: 10965
Location: Bethesda, Maryland
Joined: December 11, 2000
 They are not exactly tiny, so I imagine you could easily find a film and foil cap that is about the same size, in that value. You could probably get away with 1.0uF without losing any significant bass response, if you can confirm that the input it drives presents at least 50K ohms or better. Also, I don't categorically believe film and foil is necessarily superior sonically to a very good metallized film type, which the SG certainly is. I also like Dynamicaps and Auricaps, in that order.

 

Bypass caps smear the sound due to different speeds., posted on July 24, 2024 at 09:48:33
Ralph
Manufacturer

Posts: 4863
Location: Minnesota
Joined: April 24, 2002
 The best option is always to give it your best shot with the best coupling cap you can and call it good.

The more resolving your system is, the easier to hear that this is so. FWIW its also measurable.

 

RE: Bypass caps smear the sound due to different speeds., posted on July 24, 2024 at 12:11:12
Mendel
Audiophile

Posts: 1224
Location: GTA
Joined: January 17, 2009
 Hi Ralph.
Thanks for your response. I can't argue with your comments, you obviously know far more about amps than I do. However, the sound I am getting from the modded amp does not seemed smeared, on the contrary the amp sounds faster, and more resolving now, the way it was before seemed smeared. Obviously the new Mundorfs are more resolving than the old ones, and this more than makes up for any problems caused by the Dynacap bypass caps I guess.
I like what I'm hearing but now I'm very interested in trying the amp without the bypass caps. Is this something that I could do myself? I just can't keep lugging this 65 pound amp back and forth to my tech. I do have a very basic soldering gun, but no other equipment. Do you need a tool to suck up solder when you remove something? I assume the bypass caps are soldered in place. I think removing bypass caps would be pretty simple, but I have zero experience working inside of amps. Maybe I should just leave things be and enjoy the new sound I am getting.
The amp is definitely quieter since the mods, and power transformer mechanical hum (not through the speakers, just near the amp) has been cut way down. I put that down to the bypass caps on the power supply caps. Those are definitely staying.

 

Transformer mechanical noise, posted on July 24, 2024 at 12:43:34
Ralph
Manufacturer

Posts: 4863
Location: Minnesota
Joined: April 24, 2002
 isn't fixed by bypass caps. Its more likely that the screws that hold the end shells to the laminations were loose. That can happen over 20 years and can cause the transformer to rattle.

You don't need to solder to remove the pesky bypass caps around the main coupling caps. A set of diagonal cutters will do the job nicely. But if you're not comfortable knowing your way around the insides of the amp I'd take it to someone who does!

You'll hear no loss of highs, but you may notice it seems smoother.

Were the power supply filter caps replaced when you had this work done? The amp is old enough that would be a good idea. The 'half life' of the filter caps in tube amps ('half life' meaning about half of them are ready to fail) is about 20 years (due to heat).

 

RE: Transformer mechanical noise, posted on July 24, 2024 at 13:32:56
Mendel
Audiophile

Posts: 1224
Location: GTA
Joined: January 17, 2009
 Thanks again Ralph for the info.
Regarding the power transformer, not only are the screws tight, the previous modifications done 10 years ago included some kind of rubber padding to help reduce mechanical noise in the PT. I don't know what they did in the current modification (I don't think they did anything to the PT), but the buzz is now much reduced. So I assumed it was because of bypassing the power supply caps. As for the power supply caps, they were upgraded 10 years ago.
Regarding cutting the bypass caps, wouldn't that ruin them for future use? What if I liked the sound better with them in? They are about $30 each, seems wasteful to destroy them.
Question. Why would Rogue include bypassing the coupling caps in their upgrade package for the Stereo 90 if it is so bad to do so. That's why I did it in the first place.

 

Yeah, posted on July 24, 2024 at 14:14:46
Ralph
Manufacturer

Posts: 4863
Location: Minnesota
Joined: April 24, 2002
 If you can get them unsoldered that would be better. But again, if you're not comfortable knowing your way around the innards of the amp, take it to someone who does. Knock-on effects aren't worth the trouble!

 

DO NOT mess around with any portion of the power supply!, posted on July 24, 2024 at 18:50:28
madisonears
Audiophile

Posts: 1593
Location: midwest
Joined: September 6, 2006
 Those power supply caps can hold a significant charge, possibly lethal! All you need to do is make contact with one positive terminal and any other metal part of the amplifier to possibly electrocute yourself or at least learn the lesson that electricity can hurt. The power supply bypass caps have nothing whatsoever to do with transformer hum and probably not much to do with the sound of the amp. Leave them be.

Despite your testimony to the contrary, I have a hard time understanding why anyone, trying to improve the sound, would bypass a quality capacitor (Mundorf SIO) with a lower quality cap. It just makes no sense at all, financially or technically. Maybe they don't really like the sound of the higher quality cap and want to mitigate its contribution. I would ask the tech.
berate is 8 and benign is 9

 

RE: DO NOT mess around with any portion of the power supply!, posted on July 25, 2024 at 02:12:31
mojul
Audiophile

Posts: 39
Joined: March 10, 2020
 In my preamp I have recently changed/renewed the 1,0uF line coupling caps with Mundorf MCap Supreme Silver Gold Oil. After burn in (more than 300 hours) I have bypassed them with the Duelund JDM 0,01uF pure silver axial cap and I am very satisfied with the result.
The Duelund JDM cap has dropped in price and they are now less expensive than before (USD 54 incl. VAT).

You can read reviews about the Duelund's among many other brands at Humble Homemade HiFi. When a cheaper cap was needed in a lower quality speaker/amp or a higher quality cap in a high end speaker/amp I have used his reviews as a guideline. I think his reviews and gradings are very spot on.

https://www.humblehomemadehifi.com/Cap.html


mojul

 

Sometimes yes......mostly no., posted on August 4, 2024 at 16:55:46
Ric Schultz
Manufacturer

Posts: 848
Joined: August 7, 2000
 Thousands of people, including many many manufacturers use bypass caps. Large caps are inductive and usually do not do the speed and high frequencies perfectly. Audio Research, GR Research.....on and on have used bypass caps. Me too. As capacitors have gotten better over time.....and if you use the very top of the line ones....then you can many times get away with just one cap. You have to experiment to know what is true or not. Do not believe me or anyone. Find out for yourself. Audio is very complicated. When I see someone say something is "only this way"....then most of the time.....it is not true. Very complex is audio.

My friend has an Apogee Duetta speaker that has Vcap Odam caps in his xover.....however, every stack is bypassed by a small value copper foil V-Cap Teflon for even better sound.

 

RE: Works well in power supplies!, posted on August 5, 2024 at 10:13:08
Ralph
Manufacturer

Posts: 4863
Location: Minnesota
Joined: April 24, 2002
 But not so much coupling caps. The more resolution your system has the easier this is to hear.

 

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