[vett93]

I have an interesting experience with the size of output coupling caps for a preamp. My power amp, a Counterpoint NP100, has an input impedance of 500K. This is an easy amp to drive. Then I have a simple DIY preamp (common cathode 6SN7) with output coupling caps at 0.68uF.

If you compute the 3dB roll-off frequency of 0.68uF and 500K, you get about 0.5Hz. This is more than good enough. Sure enough, my simple DIY preamp and Counterpoint NP100 yield good bass on my JMlab Alto Utopia BE speakers. So far so good?

Then I increased the output coupling caps from 0.68uF to 5.6uF, and was surprised to find the bass is even better. So I am a bit lost because in theory they should not be any different.

To be clear, the bass with 0.68uF is very good already. But with 5.6uF caps, the bass is just noticeably better. Can someone tell me why?

[face]

What brand caps were used?

[JoshK]

I forget the whole argument, I'd have to look it up, but essentially the cap defines the rolloff frequency but the phase is affected much higher than this, maybe an octave or two.  Still if the corner frequency is 0.5hz you shouldn't have any interactions with phase.

What is the output impedance of your preamp, usually defined by the resistor to ground after the cap?  Remember that is in parallel with your amp's input Z.  So the load is probably much less than 500K.  That could explain much more of the heard effect.

[BillB]

Common cathode pres have an extremely low output impedance and a very high input.

My 6j6 is around 2k output and about 700k input.

[vett93]

Both caps are Mundorf caps. The 0.68uF is Silver/Gold & Oil and 5.6uF is MCap.

The output impedance is about 2.8K. The plate resistor is 4.5K and the plate resistance is about 7K (6SN7). The parallel of the two yields 2.8K.

But shouldn't the output impedance be in series with the amp's input impedance? In other words, the 2.8K and output caps form the combined output impedance. The 3dB freqency is when the combined output impedance equals to the amp's input impedance.

The amp's input impedance should be quite high. It is common cathode and also uses 6SN7.

[jrebman]

My theory is that you're limiting the subsonic frequencies getting to the amp so the amp's power supply is less burdened with frequencies you can't hear and therefore leaves more of it's reserves for the ones you can.

Some of the first mods for the dyna st-70s were simply putting a high-pass filter on the inputs so that less power would be consumed trying to amplify subsonic frequencies.

This assumes that the power supply on your amp isn't severely overbuilt and/or monoblocks -- which would likely be less effected by the change in output coupling caps.

That's my theory anyway.

How do you like the SGO caps?

-- Jim

[vett93]

Thanks Jim. I am not sure my description was clear. I found 5.6uF caps yield stronger bass than 0.68uF when the 3dB freq. for 0.68uF is 0.5Hz already.

These Mundorf Silver/Gold & Oil caps are awesome. They are extremely transparent and have a very refined and poised sound. They and V-caps are neck to neck in my opinion.....

[jrebman]

No, you were clear, I just didn't read correctly and thought we were talking about .56 vs. .68.  I'll go with Josh's answer then except that I think the phase can be effected as high as 10x the computed -3dB point.  I'd also bet that possibly ESR and self-inductance could figure into the ultimate answer.

I ask about the SGOs as I just had my Carina modified to remove the v-cap TFTFs in favor of the SGOs as I'm hoping they will bring a bit of that extra something that you get with an oil cap, without losing the transparency and detail of the v-caps.  If I can get my speakers put back together later today, then I'll start to be able to judge that for myself.  Thanks for theinfo.

-- Jim

[JoshK]

But shouldn't the output impedance be in series with the amp's input impedance?

No, the Zout and Zin are in parrallel.  The Zout of the gain stage though is in parrallel with the resistance from live to ground at the output connector (what works with your coupling cap to form the hi-pass), together that defines the Zout of your preamp.  Then you need to compute the Zout of your preamp in parrallel with the Zin of your amp [plus any Z in your cables that is in series, although this is usually neglible, so its ignored].  Then you have the Z that you can use to find the -3db with the requisite cap. 

I'm guessing your compute Z is too large and thus your calculated -3db isn't right, it is much higher than that.

[JoshK]

Just to be clear, in case I wasn't.  You got this part right (didn't verify but looks about right).  You are still missing the resistor on the output though, which is also in parrallel.

The output impedance is about 2.8K. The plate resistor is 4.5K and the plate resistance is about 7K (6SN7). The parallel of the two yields 2.8K.

[JoshK]

Here is an example schematic.  (It also happens to be the first tube preamp I built).



In this example, you calculated the tube's rp in parrallel with the 10K but forgot the 1M resistor. 

[jrebman]

A 1m in parallel with 10k? Or am I not "seeing" the right thing here?

-- Jim

[JoshK]

A 1m in parallel with 10k? Or am I not "seeing" the right thing here?

-- Jim

Close, it (the Zout of the preamp) would be the 10K in parallel 1M in parallel with the rp of the 5687.  I don't know off hand what the rp of the 5687 is, which is dependent on the operating point. 

[jrebman]

Josh,

I'm just saying that a 1 meg in parallel with anything in the 10k range is insignificant, and the Rp of the 5687 isn't going to change that much, unless it is closer to the 10k.  Equivalent parallel resistance is always going to be lower than the lowest value in the parallel circuit, so if the rP of the 5687 is close to, or less than the 10k, than that will explain things -- the 1m will not have any significant effect on the -3dB point.  Maybe you're saying the same thing.

Time for some more coffee
-- Jim

[JoshK]

Jim, you are correct.  I was just trying to highlight what was going on.  Sorry to belabor the point. 

Ok, instead of guessing, I did the math.  (assuming 1M) || 500K = 333K, like Jim said.  But 0.68uf into 333K load gives a -3db point of 0.7hz.  Close enough to OP's 0.5hz.

It has to be explained by the phase I'd think.

I looked back and it seems we (me) are mixing two things up.  The output Z of the preamp, which we don't really need for calculating the -3db point.  We only need to load which is the output R || with the input Z of the amp. 

[vett93]

I think you two are saying different things. But both of you have good (and valid) points.

Josh is right in that the output resistance (which is the parallel of plate resistance and the resistor value on the plate) does not play much role in determining the 3dB freq. Rather, it impacts the high freq. roll off if the input capacitance (cable and amp) is too high.

Jim is right in that the 1M output resistor does not impact much at all.

So we have a 3dB point of 0.7Hz. Even 10x of that is only 7Hz. So why do I still hear a difference between 0.68uF and 5.6uF?

[vett93]

....

I ask about the SGOs as I just had my Carina modified to remove the v-cap TFTFs in favor of the SGOs as I'm hoping they will bring a bit of that extra something that you get with an oil cap, without losing the transparency and detail of the v-caps.  If I can get my speakers put back together later today, then I'll start to be able to judge that for myself.  Thanks for theinfo.

-- Jim

These SGOs do need time to break in, not as bad as V-caps though. In other words, you cannot put new SGOs in and expect to hear the magic right away. You do get a little extra oil cap something with these SGOs, IMO. But you also lose a little micro dynamics that are unique to Teflon caps.....

My experience is with amp and preamp. They may be different with speakers....


Enjoy it!

[jrebman]

Ok, don't know and to save face I'll stop taking shots in the dark .  It's clearly not the power supply issue I first mentioned, unless that 5 or 7 hz (or .5 or .7) hits on a resonant point of the power supply, and I suppose we could be talking about either the preamp or power amp here, but still, 2 hz or .2 hz doesn't seem like it would be significant.

When all else fails I try to think of amplifiers as modulated power supplies, and if something can't be explained, I look back to the PSU.

Of course you may just be fooling yourself because of the larger cap   Just kidding.

-- Jim

[jrebman]

Yes, I expected some break-in with the SGOs, and I was sure hoping it wouldn't be as long as the v-caps .

Sorry for the confusion -- the Carina is an amp, but without the drivers in the speakers it's kind of hard to listen to it.  Well, maybe this weekend -- got totally sidetracked today with health insurance claims, mortgage refinance (is everybody in that business totally clueless?) and a dog with an upset digestive tract <grumble, grumble>.

-- Jim