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Old 26th January 2012, 12:58 PM   #1
mantha3 is offline mantha3  United States
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Default Input Impedance/Buffers

Hi,

I'm currently running a preamp that is driving 3 amps. One pair of tube monoblock amps, a 2 channel tube amp and then a plate amp that drives a subwoofer.

If I'm running the tube monoblock amps and connect the plate amp or the 2 channel tube amp I can hear a little volume drop off in the tube monoblock amps output.... I'm guessing it is just the product of the 3 amps all pulling from the same input signal....

I'm thinking of building some buffers... I was thinking it would be great to have three 2 channel buffers to isolate the signal output of the preamp to the 3 amps. I'd build 2 channel buffers just to have them be more flexable for some other use later.

I'd build them 1 at a time. for a preamp driving 3 amps with the impedance below... between a preamp and the 3 amps below... Where would the 1 buffer do the most good?

I also am finding a lot of good buffer DIY designs on this site. whats best for the buck?

My amps-

--Stereo Tube Amp - Input Impedance 270,000 ohms Input Impedance

--1 Pair 75 Watt Monoblock Tube Amps - 20,000 ohms Input Impedance

--Plate Subwoofer Amp – 22,000 ohms Input Impedance


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Old 26th January 2012, 01:13 PM   #2
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Why not rebuild the output stage of your preamp instead? This way you correct the problem where itīs at and you donīt add more sound-degrading stages.
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Old 26th January 2012, 02:06 PM   #3
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I recently finished a 6-channel Nelson Pass B1 buffer preamp and it works brilliantly. Very simple circuit and you can share one PSU over all 6 buffers without any issues.

B1 Buffer Preamp
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Old 26th January 2012, 02:08 PM   #4
mantha3 is offline mantha3  United States
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That makes sense... A lot of sense..

Would I not want to built into the ouput stage some form of buffers to isolate each pair of the 3 outputs (R/L)
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Old 26th January 2012, 02:11 PM   #5
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Why not to modify (if you can) the lower input impedances of around 20K to, say 100K or more, so not overload the preamplifier?
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Old 26th January 2012, 02:12 PM   #6
mantha3 is offline mantha3  United States
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A bit thanks for the B1 Buffer idea.. Perhaps build that type of design into my output stage of the preamp. I was looking at the B1 builds in the Pass Labs page just a moment ago.

Looks like a lot of capacatance on the power supply. What did you use for a power transformer?
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Old 26th January 2012, 03:56 PM   #7
mantha3 is offline mantha3  United States
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"Why not to modify (if you can) the lower input impedances of around 20K to, say 100K or more, so not overload the preamplifier?"

Is this just done with some reisstors?
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Old 26th January 2012, 04:01 PM   #8
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Yes, normally the first resistor at the first grid of the first tube defines the input impedance of the stuff, but have in mind the noise pickup and the Johnson noise will be increased, but with less complexity than adding stages.
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Old 26th January 2012, 04:24 PM   #9
ryuji is offline ryuji  United States
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Most circuits i see only use low impedance for the input, where an attenuator is wanted. Majority of interstage circuits i have seen use 100k and 470k values.

I myself use 100k for the grid leak on fixed bias and 470k~1 meg for cathode bias


Im curious whats your up to with your setup, you got quite a variety there:
2 monoblocks, 1 stereo amp and a plate amp. i understand the stereo and the plate, or the two mono's and the plate, but what is up with having all of them?

Last edited by ryuji; 26th January 2012 at 04:38 PM.
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Old 26th January 2012, 09:56 PM   #10
chrish is offline chrish  Australia
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Maybe he is doing something like: tube stereo for tweeters, tube mono blocks for mids and a ss plate amp for a sub? That is not an uncommon setup.

If that is the case, an active crossover will include the buffers and has the benefit of the amps not wasting power on frequencies not needed. I have used these very successfully:

Linkwitz-Riley Electronic Crossover
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