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It is currently 06 Jul 2020, 17:46

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PostPosted: 07 Apr 2020, 22:12 
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Joined: 30 Mar 2020, 00:31
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what about doubling the circuit and making it balanced from end to end? will the same transformer power it? adviseable?


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PostPosted: 09 Apr 2020, 10:00 
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Joined: 24 Oct 2010, 07:05
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Yes, it's possible to make balanced version (to be more precise - it is better to add CCS to the cathodes, do not just double circuit).Advisable? Maybe, if you know the reason for that. I can name a few:
- have balanced source and balanced power amplifier (professional gear?);
- have unbalanced source, but need to feed balanced power amplifier (long tailed stage is a very effective "convertor");
- have long interconnects or noisy environment (general useful property of balanced circuits).
As for a power amplifier to supplement the balanced preamp you can add only 1 tube to the OddWatt and convert it to fully balanced (maybe - further - change CCS in OddWatt to MOSFET-enabled and build self-balancing version).


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PostPosted: 13 Apr 2020, 20:59 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, The question of balanced comes up from time to time. Personally what I would do that would accoplish that is to use input and output transformers. One they are quite linear, two they can offer a ground lift if needed, and three they don't add the complexity and ensuing noise of more active circuitry. I have used some that way. Pick the brand and price range you like. I have used some from Edcor that were really inexpensive and looked at some from Lundahl. I figure that simpler is better and the likelihood of a final build that was satisfactory higher. I used 15K to 600CT and they were fine. I later changed the thing they were feeding to an unbalanced input and got rid of the trannies. All my other stuff was unbalanced.

Good listening
Bruce

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PostPosted: 17 Apr 2020, 17:03 
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Joined: 30 Mar 2020, 00:31
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I will find balance one day...thanks for the replies

I have a Pass B1 Buffer I am considering adding to the bypass section of the forewatt circuit. I am will power it separately with a brink as pulling 18-24v off the forewatt power supply seems challenging to me (maybe someday). The forewatt is designed for a 100k pot, the B1 a 25k but I have a 50k on it and it works fine. Will the forewatt circuit live with a 50k pot? It just needs to work, I am not overly picky about pots.


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PostPosted: 17 Apr 2020, 18:44 
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Joined: 23 Feb 2017, 02:02
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Well you can make it a balanced amp, but either your using a op amp to invert one signal and feed two of the same SRPP pre amps the same time, or your going to have to get very smart with the design.


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PostPosted: 20 Apr 2020, 08:14 
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Joined: 24 Oct 2010, 07:05
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ILoveHiFi wrote:
Well you can make it a balanced amp, but either your using a op amp to invert one signal and feed two of the same SRPP pre amps the same time, or your going to have to get very smart with the design.

I think you are overestimated the complexity of creating a balanced amplifier. Balanced input itself allows to feed signal directly as it has already had hot and cold part of the signal. If you wish to convert unbalanced input you may know an elegant solution already - OddWatt. It is SIPP (self inverting push pull). Don't take into account "push pull" and just put a CCS as the lower cathodes tail to two identical SRPP stages. That's it!

Bruce's idea of using transformers is good too, but personally I don't like it. Yes, it's easier (but not cheaper) to use transformers and some functionality is almost impossible to have in active circuits (like ground isolation), but "true balanced" idea is much better in terms of distortions, noise (including power supply noise and EMI, especially to the transformers itself) and capabilities.


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PostPosted: 21 Apr 2020, 01:54 
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No because I suspect the voltage gain to be diffrent when you use the cathode CCS to increace the impedance to approximatley very large.

The voltage gain from feeding the cathode is much higher compared to feeding the grid when the source impedance can drive the tubes proeprly.
Which is what I observed from actuall builds.

Also I was asssuming that his input signal is the unbalanced which like 100% of the audio sources are, so he needs to create an inverted version of the signal.


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PostPosted: 21 Apr 2020, 06:27 
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Joined: 24 Oct 2010, 07:05
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I really don't understand what schematic did you have in mind. Considering you mentioned impedance in cathode and "feeding the cathode" you thought about something else. I was already pointed you to OddWatt (link to the schematic is in the bottom of Bruce's posts). I can also point you to
http://vacuumstate.com/index.dna?rubrik ... 4645250299
as an example of using such layout (look into balanced versions), you may try also look to Hagerman's designs, like Cymbal (have balanced and unbalanced inputs with the same balanced driver) , Trumpet (unbalanced cartridge input with balanced amplification) and so on.
Using balanced layout with a CCS in tail to work as balanced amplifier to unbalanced input is sort to say long time standard.


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PostPosted: 23 Jun 2020, 15:07 
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Joined: 01 Apr 2020, 16:49
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Hi, I've built a Forewatt and it sounds wonderful. I have added two set of outputs as I want to bi-amp. I would llike to adjust the level of the second output to match the gain of the two amplifiers. How best to do that? Resistor divider to ground or something different?
Thanks


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PostPosted: 30 Jun 2020, 02:21 
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Joined: 24 Oct 2010, 07:05
Posts: 307
@Rytikar, the solution depends on many things. There are mostly 2 problems here:
1. Input impedance of the power amplifier, which will be connected in parallel to the lower part of the divider. It is not important if you make the pairing only to yourself and do not want to change the power amplifier in the future - then you can tune the divider in place for the next device.
2. Interconnect cables, which have some parasitic capacitance (together with part of the next device input impedance - input capacitance). The capacitance forms a frequency dependent divider with the output impedance of the amplifier. The output impedance of ForeWatt will be increased if you use an output divider, thus affecting the high frequencies. Several pages ago there is a Bruce's comment about volume controls in the input vs. output - a good explanation of the pros and cons.
In my opinion - the right place to place the divider (possibly in the form of volume control) is in the input side of the power amplifier.


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