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Old 3rd October 2006, 08:10 PM   #21
MikeB is offline MikeB  Germany
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Thanks Ilimzn, phew, much information to chew on !

About the 5k in the floating cascode, i considered using a zener instead. According to sims, the floating cascode makes the amp also insensitive to high impedance sources. In simulations distortions only rised ~10% when having a 22k in series with the input.

The wilson current mirror appeared helpful in 2 ways, better balancing and no thermal problems, as the 2 lower devices have nothing/same to dissipate. This should lower dc-offset below 0.1mv. (optimistic)
The cascoded vas will be the most interesting, i am really curious if the actual existing 2nd harmonic improves the sound or not.

The very low AC-swing in input is IMHO one of the biggest pros of symasym. This in combination with the very balanced operation of inputdiffamp should keep input devices easily in linear operation.
I once tried degeneration (47ohms) in input, but the result was degraded sound. Possible that the large nfb compensate my "lousy" layout. At least that showed me that large nfb is anything but evil.

Cascoding the drivers was just some idea...

Mike
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Old 3rd October 2006, 08:14 PM   #22
MikeB is offline MikeB  Germany
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Quote:
Originally posted by anatech

Pavel, I think we are trying to go forward and retain the good parts of the Symasym. If we lose those, there is no point. You won't get any fights there.

-Chris
Yes, i think we are conform that each change needs to be evaluated sonically and be skipped if not really beneficial.
I guess the biggest improvement for the first will be new PCB-layout.

Mike
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Old 3rd October 2006, 08:20 PM   #23
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OK, thoughts whilst lying in the bath:

This could quite easily be modular. For experimental purposes we could make a series of single layer boards for easy home fabrication, an output board that contains the zobel and big trannies, (up to three pairs for those that require them), and another that has just the basic gain stages and decoupling. This main board would just have a lot of links to the supply rails and various stages, and the assorted cascodes, CCSs etc. would stack up in a 3 dimensional style on top, and could easily be swapped out or altered as required.

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Old 3rd October 2006, 08:24 PM   #24
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Hi Mike !

Thanks for being so fast !

I didn't see your answer, and I wrote a new post at the symasym topic (that big one) !

Yes, I need 150W, or something little less, because it will be used with 100W RMS speakers in all kinds of ambience, including large ones with pretty much people.

If 150W is so complicated, 125W is easier ? I saw a schematic of Symasym 5 with 125W, but Destroyer couldn't guarantee that it works and I didn't see anybody building.

I'm not expert in SS devices, I don't know what is VAS...

Can I use MJ15024/25 at the output ?


Thanks again Mike.




Regards,


Fernando
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Old 3rd October 2006, 08:31 PM   #25
anatech is offline anatech  Canada
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Hi Fernando,
I'll give your question a stab.

As the power goes up, so do the voltages and currents required. Heat dissipated is given by the voltage x the current, so very quickly we need to change to heavier devices that do not have the same characteristics as the ones that worked so well. This is true for every part in the amp pretty much.

The Vas is the Voltage Amplification Stage. VAS.

Quote:
Can I use MJ15024/25 at the output ?
Yes, but the performance may be lower. This means less performance than a normal Symasym, but may be higher than most amplifiers.

-Chris
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Old 3rd October 2006, 08:36 PM   #26
anatech is offline anatech  Canada
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Hi busy Mike!,
Quote:
Yes, i think we are conform that each change needs to be evaluated sonically and be skipped if not really beneficial.
That would be the purpose of this schematic and board. So yes!
Quote:
I guess the biggest improvement for the first will be new PCB-layout.
Yes, a worthwhile baby step.

Hi Al,
I see two boards. Front end and current amplification stage. This allows us to run the front end from it's own supply (my intention on a better Symasym). We could then play with different output stage setups.

One big board to test concepts would work as well. This goes far beyond what I think the original concept was.

Mike, Pavel, any feelings on this?

-Chris
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Old 3rd October 2006, 08:56 PM   #27
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Quote:
Originally posted by anatech
Hi Fernando,
I'll give your question a stab.

As the power goes up, so do the voltages and currents required. Heat dissipated is given by the voltage x the current, so very quickly we need to change to heavier devices that do not have the same characteristics as the ones that worked so well. This is true for every part in the amp pretty much.

The Vas is the Voltage Amplification Stage. VAS.


Yes, but the performance may be lower. This means less performance than a normal Symasym, but may be higher than most amplifiers.

-Chris
Oh, nice, but the amp efficiency is very important to it's realiability and less power consuption / dissipation, and of course more output power.

I don't want to loose quality to have more power at the output, can I bridge two symasym? Will it degrade it's quality ?



Thank you !


Regards,


Fernando
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Old 3rd October 2006, 09:01 PM   #28
anatech is offline anatech  Canada
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Hi Fernando,
Quote:
can I bridge two symasym
Yes, but quality will suffer a little. You need an inverter for one channel. Also, the amp will "see" an 8 ohm load as a 4 ohm load. You may need more outputs to be reliable.

-Chris
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Old 3rd October 2006, 09:02 PM   #29
MikeB is offline MikeB  Germany
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Fernando, because a speaker is rated 100w you do not need to have 100w amp. It is possible to increase poweroutput for symasym, but you will need to replace all the cheap&nice 2n5401/2n5551 with exotic hard to get expensive devices, and readjust feedback compensation. Chew on that: to double the perceived volume you need ten times the power, 60watt -> 600watt ! Doubling outputpower is barely noticeable.
If you need that much power, there are also super leach amp and the Quasi's.
MJ15024/25 are nice transistors (if original) with an unique feature, they have a constant/flat ft of 7mhz for the first amperes. With an idle bias of ~110ma they should work good. It just might be tough to connect TO3s to the existing PCBs. (The drivers should be connected to the same heatsink)

Chris, Al, i think a single board prepared for all features should do it for the beginning, just to find out what to keep. As long as symasym is fed with 36v, i do not see any advantage in more outputdevices.
For a real high power version it will be necessary to use to126 devices instead of the cute to92, needing a new board anyway.

Mike
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Old 3rd October 2006, 09:09 PM   #30
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Quote:
Originally posted by anatech
Hi Fernando,

Yes, but quality will suffer a little. You need an inverter for one channel. Also, the amp will "see" an 8 ohm load as a 4 ohm load. You may need more outputs to be reliable.

-Chris

So It will loose it's greatest quality, and will be... EXPENSIVE.

Hum, I don't think so.

What's the absolute maximum power supply voltage that I can apply?

I mean, what is the maximum supply with no signal in, and at full power that the amp can handle? Because if I apply 36v, at full power it will fall for something about 32v...


Hum, good (very good) to know this power / perceived volume relation. I'm surprised.

My Beyma speakers didn't arrived yet, so I'll start working in the original Symasym 5, and let's find out if I really need more power or not when the speakers arrive.



Thanks !!



Regards,



Fernando
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