[Cheeseboy]

I have heard the Audio by Van Alstine Fet Valve Amp Ultra 550.  I liked it alot and have been considering buying one.  I can't wait to hear the new upgraded Ultra 600.  These are Hybrid amps.  AVA has a circle here. 

[Steve]

Ut oh! More equipment to try! Maybe I can get the Primaluna Dialog 3, Cary SLP 98L and the SAS 11A all in home at the same time

Hi Cap,

I can get a blemished low gain version of the B11A finished in a few days/week for you to compare, if you are getting the other two.

Cheers.

[Cheeseboy]

Steve,

Got one with a remote for volume?

[watercourse]

Cheeseboy,
I finally got the Bottlehead Paramount 300B monoblocks up and running, and I am floored at how similar the Cherry Plus and the Paramounts sound - more alike than different.
Wilson

[Steve]

Steve,

Got one with a remote for volume?

Sure don't as it degrades the sound, for several reasons.

1) Common volume control has much worse high frequency separation between channels. Lack of separation at high frequencies makes the highs sound too bright because some power is being produced on one channel that should not be there. So the Frequency response is not really flat.

Besides that, images are inferior as one channel is producing more higher frequency musical spacial information than it should.

2) Using two volume controls requires two remotes to be done right.

3) Physical size problems.

Other manufacturers compromise but I will not. I will take the very best music anytime over convenience. But that is just me. 

Cheers.

[Steve]

The Cherry line has 10K input impedance, which is why I went for a tube pre with a switchable low output impedance (90 ohms), which helps it play well with many different amps with even low input impedances.

There is the myth being perpetrated on you and others that one must have low output Z. However, this is true because the older tubes have such high output impedances; for instance 7k ohm or higher being typical.

The output Z of 90 ohms has little to do with the low frequency or high frequency response. The low frequency response has to do with the value of the output coupling capacitors or output transformer response.

From my website.

Some teach a 100:1 ratio, ............

Decreasing the ratio from 100:1 to 10:1 results in virtually no increase in harmonic distorion. For example if the total harmonic distortion at 2v rms output measures .01% (-80db) using the 100:1 ratio, changing to 10:1 raises the distortion by approximately .012% to -79db, a rise of approx 1db. Not much difference is it.

Let's check for any high frequency response advantages vs preamplifier output impedance (Z). Assume amplifier input Z of 100k ohms (Preamplifier output Z mostly influences the high frequency response.) If one uses a high capacitance interconnect cable (IC) and include amplifier input capacitance, say 250pf of total capacitance, and the preamplifier/source output impedance rises from 100 ohms to 2000 ohms, the high frequency response drops approx 0.4db at 100,000 cycles per second, and approximately .015db at 20khz.

Using a 50pf interconnect cable results in less than 0.02db drop at 100khz, and virtually zero at 20khz.

When one uses the high output impedance output tubes, like 7k ohms output Z, the high frequency response drops dramatically. For example, let's say 2k output Z and -1db down is 90khz. With 7k output Z, the response will be approximately 30khz. At 15khz the response will be down -.4db. Not good at all.

As one can see, when older high output Z tubes are used, a buffer is needed for any kind of reasonable high frequency response.

Just some information the viewers might like to know.

Cheers.

[watercourse]

So then, all things (source>preamp>amp) being equal, I change my preamp setting from low Z to high Z, and I get a limited frequency range (both high and low freqs) with the amp with a 10K input impedance.

But when I do this with my amp that has 250K input impedance, all things held constant but change only the output Z of the preamp, I get no loss of frequency response. How would this be explained if combined Z of pre/source, as well as output coupling capacitors or output transformer don't change?

Wilson

[Steve]

author=watercourse
So then, all things (source>preamp>amp) being equal, I change my preamp setting from low Z to high Z, and I get a limited frequency range (both high and low freqs) with the amp with a 10K input impedance.

Not necessarily. The lows are determined by the output coupling capacitor size. If output transformer is used, the bass will remain the same.

The highs will be limited depending upon the "high" output impedance. The higher the output Z, the lower the high frequency response. However, below about 3-4k ohms, there is little difference as I mentioned above.

As far as distortion with 10k input Z, the preamplifier/source output Z needs to be less than 2k for minimal distortion rise.

But when I do this with my amp that has 250K input impedance, all things held constant but change only the output Z of the preamp, I get no loss of frequency response. How would this be explained if combined Z of pre/source, as well as output coupling capacitors or output transformer don't change?

Well you get little if any loss in bass if you are using an output transformer, and it depends upon how large the output capacitor is. If the cap is large enough, you won't notice a change.

As far as high frequency response, what is the high impedance setting Z of your preamp?

Cheers.

[watercourse]

Steve,

Sorry I was unclear. I was posting my actual observations of differences in amps with low and high input impedances, not theoretical.

In my case, high output Z is 3.5K. Low output Z is through a tube stage using two 6DJ8s.

Hopefully this discussion will be of help to the OP.

Wilson

[Steve]

Steve,

Sorry I was unclear. I was posting my actual observations of differences in amps with low and high input impedances, not theoretical.

In my case, high output Z is 3.5K.

Hopefully this discussion will be of help to the OP.

Wilson

Yep, with 90 ohms low side and 3.5k output Z on high side you may not notice any sonic differences. But double to 7k ohms with half the high frequency response, and one will probably notice a musical difference.

90 ohms is for long, very high capacitance cables which very few use. See my above post for capacitance and FR variations.

We should get back to the intent of the string.

Cheers.

[watercourse]

Well, I'm not going to argue with an electronics designer about things I'm not schooled in, however, I can say that my empirical observations are as I've posted. The variables are held constant except for the change in output stage (no change in output trannies or capacitors) from low output Z to high output Z, and there is an audible difference on a low input Z Class D amp, but not a high input Z tube amp.
Any further particulars and I will have to contact the designer of my preamp and amps, and post their opinions.
So, I would caution that let your ears be the judge.
And I do believe this is all to help the OP learn about how to match tube pres with SS amps. So Capwkidd, just say the word and I will contact my peeps and get you more info.
Wilson

[wilsynet]

Is there such a thing as an optimal impedance ratio?  Or, all other things being equal and knowing nothing else but the input Z of the amplifier and the output Z of the pre-amplifier, is there a rule of thumb that should be used?

I've heard 10:1, 20:1, now this thread mentions 100:1.

[Steve]

Well, I'm not going to argue with an electronics designer about things I'm not schooled in, however, I can say that my empirical observations are as I've posted. The variables are held constant except for the change in output stage (no change in output trannies or capacitors) from low output Z to high output Z, and there is an audible difference on a low input Z Class D amp, but not a high input Z tube amp.
Any further particulars and I will have to contact the designer of my preamp and amps, and post their opinions.
So, I would caution that let your ears be the judge.
And I do believe this is all to help the OP learn about how to match tube pres with SS amps. So Capwkidd, just say the word and I will contact my peeps and get you more info.
Wilson

And I agree with your assessment. Maybe I was confusing in my last post. The 10k amp is going to cause changes in sound vs the high impedance input amp.

Cheers.

[Steve]

Is there such a thing as an optimal impedance ratio?  Or, all other things being equal and knowing nothing else but the input Z of the amplifier and the output Z of the pre-amplifier, is there a rule of thumb that should be used?

I've heard 10:1, 20:1, now this thread mentions 100:1.

Might check my previous post (#25) with a quote from my White Paper for expanded information.

Some have claimed 100:1 (because of their choice of hi Rp tubes they need a buffer output stage to keep the high frequency response up.). RCA Radiotron Designers Handbook recommends at 5:1 minimum. I play it safer and call for 10:1.

Hope this helps Wil.

Steve