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Old 7th February 2011, 06:16 PM   #1
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Default What's wrong with this?

I got some comments on another forum about poor design and it most likely is but what simple things would you change to make it a better design?

Sorry the schematic is poor but it probably will make sense to those of you who understand this stuff.

Thanks.
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File Type: jpg P1208507.jpg (111.3 KB, 642 views)
File Type: jpg P46 schematic.jpg (107.8 KB, 614 views)
File Type: jpg P1208508.jpg (58.9 KB, 596 views)
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Old 7th February 2011, 06:47 PM   #2
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I'm by no means an expert, but I can tell that the schematic is of poor quality. All parts values are illegible Sorry, couldn't resist.

I see an integrated amp that accepts a balanced input through a transformer. I suppose the balanced input could have been used to drive separate amplification stages right up to the output, eliminating the concertina phase splitter. This is not a simple modification though; it would require re-engineering roughly half of the amplifier circuit.

Also, I suspect that the output impedance of this circuit is actually a lot higher than your average 8Ohm speaker, so there is probably an impedance mismatch.

I guess the easiest modifications would be to change some coupling caps. I'd start right at the end, where there's an electrolytic cap that I'm sure many people on these boards have a very explicit opinion about. The first thing they'd probably suggest is to replace it and bypass it with a small value foil cap to enhance the high frequencies. The coupling caps between the phase inverter and the driver tubes could be replaced as well. Personally, I'm skeptical regarding the impact of such modifications, but two things are certain: (1) many people claim outstanding results if 'audophile' caps are used, and (2) those audiophile/phool caps can cost an arm and a leg.

But let me pass the question back to you: is there anything in particular you don't like in this amplifier?
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Old 7th February 2011, 07:58 PM   #3
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Mastodon, other than a buzz/hum in the right channel (using the RCA not balanced output), I rather like the sound it produces HOWEVER I have nothing to compare it to.

Also, I think the concern expressed elsewhere was to do with heat and how component layout inside the preamp would generate a lot of heat and create an early death for many of the electrolytic's.

I am into spending as little as possible but was concerned that maybe if I din't tackle a potential issue now I would end up paying more in the future.

I look forward to hearing opinions on the electrolytic cap and coupling caps you mentioned (are we venturing into the same world as 'power cables make a difference'? Polarized opinions usually!!)

Thanks for your prompt response.

I have scanned the schematic and penciled in what I can read and will upload it.
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Old 7th February 2011, 08:12 PM   #4
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Slightly more detailed schematic (best I can do I'm afraid)
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Old 7th February 2011, 08:41 PM   #5
tomchr is offline tomchr  United States
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Don't do any modifications to the circuit until you've fixed the hum issue. Unless there's a defective component somewhere, random component swapping will do nothing to cure the buzz.

It looks like a fairly standard push-pull design to me. I'm curious about what appears to be a gain stage after the phase splitter (2nd tube from the input).

~Tom
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Old 7th February 2011, 08:51 PM   #6
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I was hoping I would get that looked at this week but that isn't going to happen. I am hopeful that next Wednesday or Thursday a friend of a friend who knows his stuff will give it the once over.

Your comment about the design is one of the reasons I posted here. I was wondering if there was anything odd or that might peak curiosity (as well as finding out if I shouldn't risk having it on for more than a few hours at a time). I couldn't find anything to compare to this precise layout in all my searches.

Thanks for the response, Tom.
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Old 7th February 2011, 08:57 PM   #7
tomchr is offline tomchr  United States
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Don't do any modifications to the circuit until you've fixed the hum issue. Unless there's a defective component somewhere, random component swapping will do nothing to cure the buzz.

It looks like a fairly standard push-pull design to me. I'm curious about what appears to be a gain stage after the phase splitter (2nd tube from the input).

Re audiophool/boutique caps.... It's the dielectric and parasitic elements of the caps that matter. I use polypropylene caps for coupling caps due to the performance of the dielectric - extremely low loss, low dielectric absorption, and low ESR (equivalent series resistance), hence, low distortion. There are differences between brands but for foil caps of a given dielectric the differences boil down to differences in winding style and terminations. These effects are captured in the ESR and ESL (equivalent series inductance) or SRF (self-resonant frequency) of the caps.
Whether you want to use boutique caps or $0.50 metalized polypropylene caps from Digikey and other sources is up to you. One thing I've noticed is that the $0.50 cent caps tend to be better characterized than the $50 boutique caps. It's easy to measure the ESR, ESL, SRF, loss tangent, etc. if you have the right equipment. Personally, I use the HP 4194A impedance analyzer I have access to at work... Generally I find that the $0.50 caps are better characterized and have datasheets available for them but the $50 boutique caps offer no data whatsoever. They claim to work purely on patent pending magic. Sorry. That doesn't work for me. I'm a scientist and an engineer. I need claims that are backed up by valid data. Others are free to disagree with me on this.

~Tom
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Old 7th February 2011, 09:11 PM   #8
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Did you draw the schematics yourself, or did you build it exactly as in the schematics you attached? There are quite a few mistakes in the attached schematics.
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Old 7th February 2011, 09:20 PM   #9
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The schematic was sent to me from the company who sells (or sold, as it is now discontinued) the item. It was likely built in China for them and badged. The company in Canada is called Tubemagic.

It may be this is not the correct schematic
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Old 7th February 2011, 10:33 PM   #10
wa2ise is offline wa2ise  United States
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Click the image to open in full size.
As for the hum/buzz in one channel, that power transformer looks kinda close to one of the channels. Is that the channel with the hum/buzz? There may be stray magnetic field leaking from the transformer and coupling into some of that channel's wiring. You may need to change the location of that transformer.
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