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Old 27th November 2003, 08:21 AM   #1
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Default preamplifier with output transformers

If I am driving a tube amp (wave AV-8's) and want to make my own preamp, should I look at preamps that use OPTs?

I like the look of the ultrapath bp, is the 6gm8 wired in parallel? I was thinking of doing something similar with a 6c45.

OR perhaps a 26 triode with transformers?

I am not really sure of which transformers to get. Does a transformer just work on a ratio ie is a 6k to 600 ohm the same as a 240 volt to 24 volt transformer??



I am using a 24 volt B+ 6922 SRPP at the moment, I am thinking of doing something a little different, any suggestions ??


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Old 27th November 2003, 08:45 AM   #2
Colt45 is offline Colt45  Serbia
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well.. what is the input impedance you are driving?

if the input impedance is high, and you dont have 5 km of cables, you shouldnt need a low-z output (ie. transformer or CF or etc)
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Old 27th November 2003, 11:33 AM   #3
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Default Re: preamplifier with output transformers

Quote:
Originally posted by adharvey
If I am driving a tube amp (wave AV-8's) and want to make my own preamp, should I look at preamps that use OPTs?
If you want to, sure, they can work very well. Lundahls are reasonacle priced and can be sourced from Sydney.

Quote:
I like the look of the ultrapath bp, is the 6gm8 wired in parallel? I was thinking of doing something similar with a 6c45.


I like it as a general topology, but not with the 6GM8. I also wouldn't parallel 6C45's.

Quote:
OR perhaps a 26 triode with transformers?
Not tried it, but I beleive it can be very good.

Quote:
I am not really sure of which transformers to get.
Lundahls are the best value option in Oz unless you luck onto some quality preloved/NOS units. Specific model depends on what topology you choose for the amp; series or parallel fed transformers.

Quote:
Does a transformer just work on a ratio ie is a 6k to 600 ohm the same as a 240 volt to 24 volt transformer??
Vsecondary = Vprimary x turns ratio
Zsecondary = Zprimary x (turns ratio)^2
Zprimary is the output impedance of the tube stage alone.

Quote:
I am using a 24 volt B+ 6922 SRPP at the moment, I am thinking of doing something a little different, any suggestions ??
My fave design, which is cheap, good sounding, sensible gain and will drive longish cables and low Z loads well, and doesn't need OPTs is the one below (thanks Dave for drawing it). It also uses cheap, tough, available tubes.

Otherwise, I agree with Colt45, except the transformers have the advantage of galvanic isolation between stages.

The Wave8 is a good, but not great amp, so building a killer pre for it is a bit of a waste unless you'll update it later.
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Old 27th November 2003, 01:39 PM   #4
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Brett, Do you have a recommendation for the unmarked resistor and cap?
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Old 27th November 2003, 10:13 PM   #5
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Quote:
Originally posted by maynardg
Brett, Do you have a recommendation for the unmarked resistor and cap?
For the R at the input, anything from 10k - 250k depending on what the previous stage is happy driving, say 200k.

The cap should be sized as small as possible to give the -3dB corner you want depending on the input Z of the next stage. Smaller caps usually sound better.

C = 1/(6.28 x Zin x f3) in F
I usually aim for 1Hz.

There should also be a shunt resistor after the output cap of 1M. Not strictly neccessary, but reduces thumps if plugging the output whilst powered up,
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Old 30th November 2003, 11:17 AM   #6
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I have a pair of Wave 8. I use them with a simple passive preamp.
The input sensitivity is 500mV so a passive pre works fine.
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Old 1st December 2003, 01:05 PM   #7
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These guys do a nice pre-amp kit with output transformers (I have one)
http://www.worldaudiodesign.co.uk
They sell the transformers seperately, and they have a sale on right now.
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Max
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Old 2nd December 2003, 12:57 AM   #8
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i couldn't agree more the recommendation for 12b4. i just built this guy and what a wonderful preamp! in my system, it's very revealing and has sparking highs.

i'm using b+ of 260v. plate of 12k, running at about 15ma. biasing at 7+v. i'll try a different operating point later.

DonJuan
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Old 2nd December 2003, 01:10 AM   #9
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Hi,

Quote:
biasing at 7+v.
You meant -7?

Cheers,
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Old 2nd December 2003, 08:55 AM   #10
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Quote:
Originally posted by DonJuan
i couldn't agree more the recommendation for 12b4. i just built this guy and what a wonderful preamp! in my system, it's very revealing and has sparking highs.

i'm using b+ of 260v. plate of 12k, running at about 15ma. biasing at 7+v. i'll try a different operating point later.

DonJuan
Glad you like it. I think it's a ripper, that's why I keep recommending it. Be interested to hear your views with a higher bias.
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