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M2 inspired balanced output preamp idea
M2 inspired balanced output preamp idea
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Old 17th February 2019, 08:10 PM   #11
R-K Rønningstad is offline R-K Rønningstad  Norway
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You could easily convert that circuit to a B1 Rev. 2.
Q1 and Q4 can be omitted, just install jumpers between the emitter and collector of each of them. Omit also R1, R2, R7 and R8.
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Old 17th February 2019, 10:17 PM   #12
woofertester is offline woofertester  United States
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Originally Posted by Mark Johnson View Post
Calculate the output impedance of the preamp. How much capacitance can it drive without damaging the HF response? How many feet of interconnect cable is that? What happens when you connect a solid state power amp whose input impedance is 10K ohms, at the far end of a 5 meter interconnect cable?
Good questions. I will connect one of these up and see if I can device some torture tests.

I did perform an output impedance test a long time ago but have forgotten the results. I will re-do and report here.
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Old 17th February 2019, 10:48 PM   #13
woofertester is offline woofertester  United States
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Originally Posted by R-K Rønningstad View Post
You could easily convert that circuit to a B1 Rev. 2.
Q1 and Q4 can be omitted, just install jumpers between the emitter and collector of each of them. Omit also R1, R2, R7 and R8.
That is a great idea. I may buy some unpopulated kits and assemble them as a B1 R2. Thanks.

One attraction of the assembled boards is that they throw away 1/2 of the rail voltages. Great for a front end buffer driving a voltage gain stage. You can use the OPS rails.
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Old 18th February 2019, 08:33 AM   #14
indra1 is offline indra1  Indonesia
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Originally Posted by woofertester View Post
Not autoformer...
Use of a level matching transformer as an autoformer can have performance advantages compared to the normal transformer mode with regards of parasitics and losses.
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...You could call this an unbalanced to balanced converter with voltage gain. ...
For best result, an unbalanced to balanced transformer usually use a different winding technique and/or shielding compared to a regular level matching transformer.

I encourage you to take more measurements on the PC 600/15k and perhaps other alternatives since key performance factors will somewhat deviate from the M2 due to different mode of utilization. And for output, I woul think the Whammy output stage will serve you much better instead of the tiny buffers.
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Old 18th February 2019, 07:10 PM   #15
R-K Rønningstad is offline R-K Rønningstad  Norway
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Regarding the JFET input Cascoded Buffer Preamp Board use of Toshiba 2SK246/2SJ103, C2240/A970 699970145561 | eBay I would not know if the JFETs are actually 2SK246/2SJ103, but if one for one reason or another want to use 2SK170/2SJ74 instead that should be a piece of cake.
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Old 18th February 2019, 08:29 PM   #16
woofertester is offline woofertester  United States
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Originally Posted by indra1 View Post
Use of a level matching transformer as an autoformer can have performance advantages compared to the normal transformer mode with regards of parasitics and losses.

For best result, an unbalanced to balanced transformer usually use a different winding technique and/or shielding compared to a regular level matching transformer.

I encourage you to take more measurements on the PC 600/15k and perhaps other alternatives since key performance factors will somewhat deviate from the M2 due to different mode of utilization. And for output, I woul think the Whammy output stage will serve you much better instead of the tiny buffers.

Hello Indra1. Nice to hear from you. Do you have a link to some technical info about these different transformer types?

I took your advice and had a look at the Whammy OPS. It looks like the same transistors from the BA3 preamp but configured as a follower. I have some of these and can give it a try to compare to the buffer boards.

I did order two other different configurations of the EDCOR line transformers to characterize.
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Old 18th February 2019, 08:33 PM   #17
woofertester is offline woofertester  United States
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Originally Posted by R-K Rønningstad View Post
Regarding the JFET input Cascoded Buffer Preamp Board use of Toshiba 2SK246/2SJ103, C2240/A970 699970145561 | eBay I would not know if the JFETs are actually 2SK246/2SJ103, but if one for one reason or another want to use 2SK170/2SJ74 instead that should be a piece of cake.
One attraction of the buffer board is that it is $11 populated for two channels. A quad of Toshiba 2SK170/2SJ74 from Punky Dawgs is $65 and then you have to add all of the other parts and PCBs. So,.. just investigating a lower cost possibility.
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Old 18th February 2019, 09:03 PM   #18
R-K Rønningstad is offline R-K Rønningstad  Norway
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It’s the 2SK246/2SJ103 though, but those are more than good enough for rock’n’roll!
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Old 18th February 2019, 09:23 PM   #19
indra1 is offline indra1  Indonesia
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Originally Posted by woofertester View Post
... Do you have a link to some technical info about these different transformer types?...
I have only scratched the surface on the subject, but it seems common knowledge that a phase inverted secondary is somewhat worse compared to in phase. As an example, Lundahl LL1660s employs a "internal Faraday shields to improve balance in phase splitting interstage applications" compared to the LL1660 which is the regular interstage/line output. Here in Papaland, you'd probably get way more from ZM, people used to call him Choky, or talk to tube forum guys deep into iron if you need technical details like 45, Bud Purvine and Wavebourn.
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Old 18th February 2019, 10:36 PM   #20
woofertester is offline woofertester  United States
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Originally Posted by indra1 View Post
I have only scratched the surface on the subject, but it seems common knowledge that a phase inverted secondary is somewhat worse compared to in phase. As an example, Lundahl LL1660s employs a "internal Faraday shields to improve balance in phase splitting interstage applications" compared to the LL1660 which is the regular interstage/line output. Here in Papaland, you'd probably get way more from ZM, people used to call him Choky, or talk to tube forum guys deep into iron if you need technical details like 45, Bud Purvine and Wavebourn.
Thanks. That gives me some ideas. If there are some parasitic paths that cause imbalance in one transformer, it may be possible to use two separate transformers per channel and also add a mechanism for twiddling the balance between halves. That will add complication, parts and cost.

One of the ideas is to cobble together an instrumentation amp that can measure the goodness of the balance between halves.

cheers
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