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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 29 Oct 2018, 19:28 
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Yes this thread has be some very popular. And is dead simple to build a 4S or 4SU.

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PostPosted: 01 Nov 2018, 20:29 
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Joined: 19 Oct 2018, 15:30
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Location: Montréal, Québec
I look at the output impedence specs and realize it might not fit any power amp I have. Problem is I already bought everything to build this preamp (already had the exact same power supply laying around)

So, how do I know if the output impedence of the pre fits the input inpedence of my amp ? Is there a way to adjust it to the specs of my amp ?

Thanks !


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PostPosted: 02 Nov 2018, 03:40 
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Joined: 20 Apr 2015, 01:30
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I built the 4S Universal, and used it very successfully with a number of different DIY tube amps. In order to use the circuit with a couple of solid state amps that I own with an input impedance of 10K Ohms, I modified the 4S with a 12AU7 cathode follower as per this thread:
http://diyaudioprojects.com/Forum/viewt ... f=9&t=5186
I am in the process of debugging it in that other thread, as I have an issue with hiss that I didn’t have with the original 4S Universal. Aside from the hiss, it works well with my Pass Amp Camp Amps, and with my Audiozone Amp-1. The 4S Universal did not sound good with these two SS amps without the cathode follower added. That said, 10K Ohms is on the low side even for SS amps, so YMMV.


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PostPosted: 02 Nov 2018, 11:06 
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Joined: 06 Apr 2009, 10:08
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Location: US Pacific Northwest
Gab wrote:
So, how do I know if the output impedence of the pre fits the input inpedence of my amp ? Is there a way to adjust it to the specs of my amp ?
The answer to your first question is a simple check. In order to work well, the amplifier being driven should have an input impedance of at least 4 times the output impedance of the preamp. The peak output impedances of the 4SU can be found in the table here: http://www.diyaudioprojects.com/Forum/viewtopic.php?f=9&t=4628&start=61

Please note that the peak output impedance numbers in the table depend on the volume setting. They start low, grow to maximum at about 80% volume, then fall off again as volume goes to max. A discussion of this phenomenon can be found here: http://www.diyaudioprojects.com/Forum/viewtopic.php?f=9&t=3266&start=121

An option to lower your output impedance is to move the volume control to the input of the preamp. Then the output impedances become fixed and much lower than the peaks shown in the above linked table. When built in this fashion, the output impedances (with cathode bypass) are approximately as follows:

    Tube Zo
    --------------------
    12AU7 ≈ 11.5kΩ
    12AV7 ≈ 13.4kΩ
    12AT7 ≈ 20.0kΩ
    12AY7 ≈ 23.1kΩ
    12AX7 ≈ 38.7kΩ

The answer to your second question is more complicated. The output impedances of the preamp are set by the tube characteristics and the component values. As such they really isn't much room for adjustment of the preamp itself. The answer to your concern however (as tizman has pointed out above) is to include a buffer between the preamp output and your amplifier. This was the whole point of the thread linked above. The cathode follower has an output impedance of ≈1.4kΩ which is low enough to drive virtually any amplifier.

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PostPosted: 02 Nov 2018, 14:25 
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Joined: 19 Oct 2018, 15:30
Posts: 35
Location: Montréal, Québec
Thanks for your detailed answers ! I really appreciate your write ups btw !

I think I don’t have any choice but going the carhode follower way since the amp I want to drive has an input impedance of 25k ohms (yamaha m-50). I’ll carry on my reading in order to avoid the kind of mistakes I have made. I guess it is part of the learning process ! This is my first tube project Matt, do you realize that of all the diy tube projects out there, I decided to start with yours ? :beerchug:


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