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Old 12th November 2009, 04:49 PM   #1
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Default High Zout tube preamps with ss power amps.

I wonder how many people use tube preamps with high output impedance driving low input impedance ss power amps and are quite happy with the resulting HF response. By that I mean they don't feel that that the HF is rolled off or 'soft'.

I've been playing around with a preamp with an output impedance of around 8 K ohms. On my sim it shows the following performance with two different power amps:

amp1 : 77pF ( interconnect cable ) + 150 pF (amp input )= 225 pF total capacitance

Rin = 10 K ohm

20 Khz response = -0.07 dB / -7.2 deg

amp2 : 77pF ( interconnect cable ) + 275 pF (amp input )= 352 pF total capacitance

Rin = 20 K ohm.

20 Khz response = -0.271 dB / -14.2 deg

So it appears that if we keep the interconnect and other capacitances low , the high output impedance should be OK.
In a majority of cases the ss amp input impedance will be typically around 10 K ohms or greater. If it's DIY this is easy with most circuits.

It appears that we might not need a cathode follower stage in many cases. Distortion figures might vary with the different loading on the input stage but I wonder if that would make a significant audible difference (?).

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Old 12th November 2009, 05:01 PM   #2
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Some SS power amps have a low pass network, with a shunt capacitor near the input terminal. The Naim designs do this, for example.

These may suffer more seriously from HF loss.
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Old 12th November 2009, 05:15 PM   #3
AndrewT is offline AndrewT  Scotland
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it's not the input resistance that is the HF problem.
It's the RF filter components.
We usually aim for the RC of the low pass filter to be between 0.3us and 1.5us.
Your 8k source impedance restricts the cable and amplifier capacitance to between 37pF and 190pF.
Cable alone causes excessive RF filtering. The added capacitance inside the power amp pulls the filtering effect inside the audio band.
Rs=8k and Cload >=300pF will be noticed by the majority of listeners.
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Old 12th November 2009, 05:37 PM   #4
SY is offline SY  United States
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Moreover, depending on the preamp design, the lower loading could significantly reduce gain and increase distortion. For example, an open loop amplifier with output taken off the plate will probably have that plate load halved by connecting it to the SS amp. That means3-6dB or gain loss, and probably double the distortion.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 12th November 2009, 10:11 PM   #5
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I use a pre amp with a ECC83 but sit the pre amp on top of the amp so I only need a very short interconnect lead.

For my hybrid tube/MOSFET amp I use a ECC82 in SRPP mode to lower the impedance.
Murton-Pike Systems PCBCAD51 pcb design software. http://www.murtonpikesystems.co.uk
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Old 13th November 2009, 01:47 AM   #6
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"........significantly reduce gain and increase distortion........."

Yes I was aware of that. I was considering a preamp which can give enough voltage to drive the power amp with a 1 volt input signal and have acceptable levels of distortion. Using a CCS on the anode and a low external impedance ( load ) , it might be similar to using a resistive load on the anode. At least the sim indicates that.
Last night I completed the test board for a single tube preamp . Will try to test it today and see how it fares.
On many power amps I've seen the input RF filter cap values vary from about 33pF to 470pF. This would also depend on the over all input impedance and the series input resistor.

I just wanted to find out if we have all these parameters under control (or known ) and we use a high Zout preamp , would it sound good enough assuming we take care of the interconnect capacitance. I will have to check out a Cat5e type DIY IC also. Does anyone know what capacitance that has per meter ?

By the way I measured the capacitance of an average RCA socket as 5 pF. Will have to check that on some expensive RCA connectors that I have.

This preamp uses a CCS , unbypassed cathode resistor and has an ac heater supply. Simmed Zout is just about 8 Kohms. I'll be trying out a resistor loaded anode and a CCS loaded anode. I provided space on the pcb to take either load.
The CCS uses a MJE350 and BC556B . The power supply is RC filtered with a capacitance multiplier following that. Anode load is further filtered with a typical split resistor and capacitor to ground.

I have a few commercial power amps and some DIY power amps to try it on.
I can change the RF filtering on the commercial amps if required . I'll decide what to do after checking out the response with the preamp attached.
Will be doing some listening tests this weekend with a couple of friends and a few bottles of wine ( DIY too !).


Last edited by ashok; 13th November 2009 at 01:50 AM.
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Old 13th November 2009, 03:33 AM   #7
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I recently installed ferrite beads and 1nF ceramic caps on my SSamp's inputs (the local radio station was getting quite annoying!). Doesn't sound any different. Preamp has 12AU7 CF outputs, Zo around 600 ohms let's say.

The amp itself has 10kohm "base leak" resistors.

I can't imagine using a preamp where the output impedance is so high (like 12AX7 high). HF response doesn't really matter, I would hate that simply because gain would vary so much, let alone distortion.

Stuff like 12B4 plate is almost 12AU7 cathode scale and no big deal; 6DJ8 follower would be quite excellent, as well as other high-Gm types.

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Old 13th November 2009, 05:57 PM   #8
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Default Quick preamp test.

I tested the tube preamp in a hurry tonight. Zout is high and appears to be a bit higher than 8K ohms. Response with a 8.7K ohm load and 320pF capacitance is -0.6dB at 20Khz ( measured). Distortion with a CCS is 0.03% at 2 V rms ! With a resistive load at the anode the distortion is much higher. With a CCS I see practically only the second harmonic the higher orders being very low . With a resistive anode load the third fourth and fifth are also significantly present though they keep decreasing in amplitude. So CCS is the way to go.

I rigged it up with one power amp and I must say that it sounds very nice. I can't play it loud as it's midnight here. Tomorrow or Sunday will be the day for regular listening sessions. I don't hear any loss of HF and it doesn't sound 'bright'. Voice sounds very good and bass is good too. But I can't say too much as I am playing it quite soft.

So my question is answered. High Zout preamps sound fine if we operate them within reason. All parameters must be known , load impedance , capacitance and drive voltage required. We can eliminate the cathode follower and reduce the number of active devices in the chain.

This preamp appears to get up to about 10 V rms before the distortion rises rapidly. I need less than 2 V rms to drive the power amp into clipping. Can't wait to check it out tomorrow.
Now I have to crash out. It's been a long day !

Last edited by ashok; 13th November 2009 at 06:02 PM.
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Old 14th November 2009, 11:43 AM   #9
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Default Listening tests.

I spent 6 hours listening to the high Zout preamp.I used a Rotel RA-970BX integrated amp. This one is modified. The input goes directly from the input selector to the volume pot which goes directly to the power amp via a film cap. I removed the original Black Gate which was not as good as the film cap.
So it is just a power amp now. The bypassed preamp and tone control circuitry degrades the sound.

This is a very good sounding preamp. No apparent loss of highs and seems to be good right across the bandwidth. Great transients and clean HF.

Compared to an older preamp I had which has a CF output , this premp is much better. Voices sound really nice. The Rotel sounds better via the pre amp. This premp transforms the amp altogether. I've seen that sort of thing happen many times in other systems.
I kept all interconnects not longer than 2 feet each. There were several cables.
Cable from DVD player coax digital output to DAC coax SPDIF input. DAC analog output to passive Alps volume control. Cable from Alps to tube preamp and cable from preamp to power amp input. Three cables carrying the analog signal !

Now I need to compare this one with my 12B4A preamp which is in storage. I'll have to find a power supply for it.
The 12B4A also does not have a CF output stage. That preamp used to sound quite nice but the comparison is going to be interesting. The CF stage can be eliminated from the signal path if possible. Of course long interconnects can't be used .....unless one uses cables with very low capacitance .

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Old 14th November 2009, 03:50 PM   #10
Minion is offline Minion  Canada
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My DIY guitar amp uses a tube preamp and a solid state poweramp , I put a unity gain high impedance buffer between the tube stage and the power amp so I get a low impedance going into the poweramp .... it works well for this application ...
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