[mlundy57]

I am trying to understand the important aspects of matching preamps to power amps, especially passive preamps. I have a few questions but I will start with this one:

What characteristics of the preamp and power amp are the most important to insure best performance?

Mike

[G Georgopoulos]

You're not specific enough tubes,solid state,hybrids,it's not matching, most rather drive dynamics,passive preamps dont have the edge active preamps have...

[FullRangeMan]

I am trying to understand the important aspects of matching preamps to power amps, especially passive preamps. I have a few questions but I will start with this one:

What characteristics of the preamp and power amp are the most important to insure best performance?

Mike

IMO it would be the right impedance.
According the old rule thumb the preamp should have output impedance 10x more than the power amp.
My 2 cents.

[mlundy57]

You're not specific enough tubes,solid state,hybrids,it's not matching, most rather drive dynamics,passive preamps dont have the edge active preamps have...

In this particular case I'm looking at matching a Dodd Audio battery powered tube buffer with a power amp to get the least noise, best clarity and best dynamics. The type of power amp (tube, solid state or hybrid) doesn't really matter but performance does. The power amp would be driving 94dB efficient open baffle speakers that roll off at 200Hz. Frequencies from 200Hz and down are handled by stereo open baffle servo subs, each with it's own plate amp.

I have tried mating the tube buffer to the power amp section of my NAD C372 stereo amp but the results were not favorable. When using the C372's preamp there was very little noise when nothing was playing. When music was playing the sound was clean, clear and distinct. However, when the tube buffer was handling the preamp duties, there was quite a bit of noise when nothing was playing and music was not as clean, clear or articulate. The sound was more muddy.

The C372 has 150wpc (massive overkill but what I have) with a 50 kOhm input impedance. I don't know if these particular specs matter or if there are others that are more important. What I do know is that this is not the performance this tube buffer is known for so I am trying to figure out what is not playing well together and why.

Mike

[mlundy57]

IMO it would be the right impedance.
According the old rule thumb the preamp should have output impedance 10x more than the power amp.
My 2 cents.

10x more than the power amps output impedance or its input impedance?

Mike

[FullRangeMan]

Example: Pre= 100K ohms, Power=10K ohms.

[FullRangeMan]

The useful Decware site always help in these moments:
http://www.decware.com/paper55.htm

[G Georgopoulos]

FRM is top notch on this! ,but i think he's trying to tease you..

[mlundy57]

The useful Decware site always help in these moments:
http://www.decware.com/paper55.htm

Good information. Now I need to find the specs on the Dodd tube buffer

Thanks

FRM is top notch on this! ,but i think he's trying to tease you..

Quite possibly but the page he directed me to helped me understand part of the picture. Now to dig deeper. Why did I sleep through my physics classes 40 years ago?? 

Mike

[G Georgopoulos]

Quite possibly but the page he directed me to helped me understand part of the picture. Now to dig deeper. Why did I sleep through my physics classes 40 years ago?? 

Mike

Actually,I dont agree with the page he directed you,because it talks about sensitivities and clipping,preamplifier synergy (dynamics) is not about voltage levels but impendaces values these enhance dynamics...a source must have a low z out and a pwramps high z in...

[RDavidson]

It's a little strange that the Dodd is noisy compared to the NAD's preamp section (though the NAD's preamp is likely pretty quiet as it is class A). To be sure, you removed the pre-out / main-in links and connected the Dodd to the main-in input, correct?

If that's the case, and it's still noisy, I'd look at the tube. The Dodd is known for being super quiet. It is very doubtful you have an impedance issue. That's the main point of a buffer. It provides a nice, low, output impedance to the amp.

FRM, you got your wires crossed. Reread the helpful and comprehensive Decware link you provided. No worries, I get the whole output/input impedance thing mixed up too and had to read the Decware link to confirm what I thought was correct.

"All sources and preamps have what is called "output impedance". Think of it as the units ability to drive difficult loads, like might be experienced with super long interconnects, or obnoxious amplifiers.

On the flip side, all amplifiers have what is called "input impedance". Think of it as the difficulty level the amplifier imposes on the preamp or CD player.

As a general rule, the lower the output impedance is, the better it will drive difficult loads. And, the higher the input impedance is the less difficult it is to drive. A good example of a happy situation is a preamp with an output impedance below 1000 ohms driving an amplifier with an input impedance of 100,000 ohms."

[RDavidson]

Actually,I dont agree with the page he directed you,because it talks about sensitivities and clipping,preamplifier synergy (dynamics) is not about voltage levels but impendaces values these enhance dynamics...a source must have a low z out and a pwramps high z in...

George. I disagree. Impedance isn't the only factor at play. Voltage and sensitivity are the other half of the equation....just as Decware explains.

[mlundy57]

It's a little strange that the Dodd is noisy compared to the NAD's preamp section (though the NAD's preamp is likely pretty quiet as it is class A). To be sure, you removed the pre-out / main-in links and connected the Dodd to the main-in input, correct?

Correct. I removed the links that came with the NAD and used the same Audioquest Forest interconnects for both preamps.

If that's the case, and it's still noisy, I'd look at the tube. The Dodd is known for being super quiet. It is very doubtful you have an impedance issue. That's the main point of a buffer. It provides a nice, low, output impedance to the amp.

After reading the Decware paper I would agree. Voltages should not be an issue, my DAC has an output of 2V while the NAD power amp has an input sensitivity of 770mV. Impedance also looks good with the Dodd having an output impedance of 130 Ohms while the power amp's input impedance is 20kOhms (my earlier post of 50kOhms was actually for the NAD's preamp).

The tube I am using in the Dodd is a Reflektor 6H30-DP (toggle switch set at 6V).  Should I get a new one of these tubes or would a different tube work better?

Mike

[Early B.]

OK, technical specs aside, there's more to the story.

Let's take a typical scenario where the output voltage of your source is 2 volts, the input sensitivity of your amp is 1 volt, and you have an impedance match between the pre and amp.  In this typical example, there are a zillion preamps that qualify purely on specs and within any given price range. What else do you need to look for in matching the amp to the pre?

I'd also like to hear your thoughts about the added value (or not) of purchasing an amp and pre in the same class from the same manufacturer. Since they are usually designed to be mated with one another, is there better "synergy" due to same build quality, same parts (caps, wire, etc.), same design philosophy, etc.???

[G Georgopoulos]

George. I disagree. Impedance isn't the only factor at play. Voltage and sensitivity are the other half of the equation....just as Decware explains.

Mr Davidson,it's all about audio before clipping, the only thing that enhances current and voltage is impendance ,more voltage does enhance dynamics but then you get clipping,withing the voltage amplifying range,the impendance enhances without clipping,trust me on this ...

[Speedskater]

Example: Pre= 100K ohms, Power=10K ohms.

This must be a typo.
In a typical solid state system:
Pre-amp output impedance 100 to 200 Ohms.
Amplifier input impedance 10,000 to 100,000 Ohms.

Compatibility might be a better word choice, rather than 'matching'.

[FullRangeMan]

This must be a typo.
In a typical solid state system:
Pre-amp output impedance 100 to 200 Ohms.
Amplifier input impedance 10,000 to 100,000 Ohms.

Compatibility might be a better word choice, rather than 'matching'.

You are right I invert the values.
Sorry the fail.

[Speedskater]

You are right I invert the values.
Sorry the fail.

If that's the biggest mistake that we make today, it well be a great day.

[Rusty Jefferson]

...The tube I am using in the Dodd is a Reflektor 6H30-DP (toggle switch set at 6V).  Should I get a new one of these tubes or would a different tube work better? ....

In my experience, that particular tube can be problematic to get quiet enough even for a linestage. Perhaps you can try one that's  been graded for low noise/microphonics. The 6h30 is not a direct replacement for 6dj8 in all circuits either. I thought these buffers use 6dj8.

Also, i think these buffers were available  as kits for DIYers. (?) If that's the case, you'll have to check the build.

Sometimes, just adding a component and interconnects will add noise. A grounding issue between the amp section of the integrated and buffer. Can you borrow a stand alone amplifier?  Then, all the normal stuff to help reduce noise like everything plugged into the same receptacle, etc.

[mlundy57]

Another suggestion that was made outside this discussion was to try a different source. I have an OPPO 105D in my HT system so I disconnected the DAC from input 1 on the Dodd and moved it to input 2. Then I connected the OPPO's stereo analog out connections to input 1 on the Dodd (just to keep things consistent) and, nothing. Absolute dead silence. No noise whatsoever. I switched to input 2 on the DODD (the DAC) and, no noise as long as both the DAC and computer were off. When I turned the DAC on there was the tinest bit of noise. However, when I turned the computer on, major noise. I switched back to the OPPO on input 1 and again no noise whatsoever. Back to input 2, major noise.

So all the noise is coming from the computer. My guess as to why it is worse with the DODD than the NAD preamp is that the DODD is doing a much better job of getting out of the way and passing more of what it gets onto the power amp than the NAD preamp does. I could be wrong but that seems logical.

Also, through the OPPO, not only is the noise gone, but all the clarity and dynamics are back. Now we're cooking with gas.

I have an iFi USB power and am using it in combination with my iFi iDSD DAC with the Dell server. It doesn't reduce the noise.  The next thing I'm going to do is take the SoTM sound card out of my C.A.P.S. V3 Carbon and put it in the Xeon based Dell workstation I'm using as a server. Then if that doesn’t clean things up enough I will probably add a P.I. Audio miniBUSS and/or an Up Tone Audio Regen and an Audioquest Jitterbug. Not sure which to try first. 

Mike