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Balanced Differential Linestage with Low Mu - perhaps DHT ?
Balanced Differential Linestage with Low Mu - perhaps DHT ?
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Old 7th November 2017, 08:47 AM   #1
RhythMick is offline RhythMick  United Kingdom
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Default Balanced Differential Linestage with Low Mu - perhaps DHT ?

Although he likes my autoformer-based passive pre, my brother is reluctant to give up his active line stage pre. The problem is that he never gets the volume knob past about 25%, so he's throwing away most of the input signal only to amplify what's left.

He acknowledges it loses bandwidth compared to my autoformer (particularly at the bottom end) but he likes the "drive" it brings. I don't hear it, I think he likes the colouration. It's a beautiful hand-crafted balanced differential pre using 4 x 12AU7 valves, which have a mu of about 17.

So I have started thinking about building him a line stage with a much lower gain. He doesn't need ANY gain - I'm driving the same power amps (45 output tube) into the same speakers (Lowther DX3 + subs) with my autoformer.

I was wondering about a balanced differential line stage based on low-mu DHTs, perhaps the 71A (mu=3). I couldn't find anything in a brief search this morning. Has something like this been done and are their circuit diagrams I could look at ?
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Old 7th November 2017, 09:03 AM   #2
SemperFi is offline SemperFi  Wake Island
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Ive done a pre with 3A5s. Not very low mu but dht. My current pre can take 6AS7 which is low mu. Precently I have 6BX7 in there b/c I need some gain. This pre can take all the octals with same pin out so 6SN7 and 6SL7 as the highest gain tubes. But I prefer the lower gain of 6BX7 and 6BL7.
Sorry not much help. But my point is yes u can build a pre with low mu tubes.
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Old 7th November 2017, 09:08 AM   #3
SemperFi is offline SemperFi  Wake Island
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There is a thread regarding 6V6 in preamp. For some reason I havent read it myself yet. But Id think it fits your needs.
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Old 7th November 2017, 11:03 AM   #4
RhythMick is offline RhythMick  United Kingdom
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Note the requirements for BALANCED, input and output
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Old 7th November 2017, 11:14 AM   #5
DF96 is offline DF96  England
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Using low mu valves in an LTP requires the use of a CCS tail. This is because the degree of unbalance in an LTP with resistive tail varies like 2/mu (IIRC).
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Old 7th November 2017, 02:06 PM   #6
SemperFi is offline SemperFi  Wake Island
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Quote:
Originally Posted by RhythMick View Post
Note the requirements for BALANCED, input and output
oops sorry. It even says 'Balanced differential' so there really should be no ambiguity...
I'm usually so good at reading headlines (and very little else) so dunno how I missed it.
71As are not that cheap, and they're probably not tightly matched, you wont buy tens of them to find two matched sets, or..? So DF96's tip is definately a must.
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Old 7th November 2017, 03:17 PM   #7
audiowize is offline audiowize  United States
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Does the linestage have to be differential? You could use two 12AU7s to make four cathode followers and end up with a balanced active zero gain buffer.
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Old 7th November 2017, 03:25 PM   #8
RhythMick is offline RhythMick  United Kingdom
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Yeah I was starting to think that way, reading about the Balanced Aikido.

Balanced Aikido

I fancied doing it with DHTs, but as has been said finding matched sets will be difficult and they are not cheap.
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Old 7th November 2017, 04:03 PM   #9
RhythMick is offline RhythMick  United Kingdom
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I've never used a CCS before. Could someone point me in the right direction for some good reading to get a good understanding.

Are they best bought and built as kits, like Rod Colemans filament regulator ?
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Old 7th November 2017, 05:11 PM   #10
MagicBus is offline MagicBus  Greece
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I' ve found great assistance for my current project here:http://waltjung.org/PDFs/Sources_101_P1.pdf http://waltjung.org/PDFs/Sources_101_P2.pdf
Also a lot of info in Merlin Blencowe's book.
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