Class A amplifier project using ready built circuits.

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Hi, I have just joined the website and have some issues to put out for thought. I am also writing this post to help others in their builds, and for me to seek help and experience too. Over several years I have been buying amp circuits from ebay, which are clones of krell KSA50 boards.

I should re-phrase that, they are boards designed well, from the circuit schematic of the Krell KSA50-2. They are good quality and sound very good. I should add, previous amp circuits I have tried and are very good are the Naim clone NAP135. The nap135 were nowhere near as good as the ksa50.

There are issues with the ksa50 boards with respect to user construction. They must be placed dead flat on the heatsink using pads and paste. I have lost many expencive speaker drivers as well as many amp circuits by being less than critical on the heat tranfer to the sinks.

I am at a point with the system build where I have 4 x current psu's, 4 x voltage psu's 4 x ksa50 amp circuits, a homemade preamp using Naim 532 MC phono boards, alps pot and selector switch, kendeil DC supply caps and outboard psu.

The speakers are tannoy 12" in 150 Liter Wilmslow cabs. I have made a record turntable using a solid oak plinth with an aluminium and perspex platter and a Mitchell RB300 tonearm with Audio Technica AT-F7 cartridge and outboard brushless DC motor.

OK, this system has problem areas which I am going to discuss. This project is very time consuming so I tend to take a while to rectify issues. Here are a couple of issues.

1. Current power caps are so large, the power transformers are causing charging noise every so often, this is because the caps are Kendeil 100,000 uf 50V as the current section runs at +-45VDC. The transformers are 680VA x 4. The voltage power supplies have 250VA with 5600uf Kendeil caps 63V, as the voltage section runs at +-55VDC.

2. The preamp has a strange issue of either oscilation, or feedback when the turntable is connected and the vulume turned up above half way. The speakers sound like they are putting out 50Hz as the driver excursions ramp up to full swing with increasing volume position. Strangely, if I unplug the tonearm connections it doesn't happen. Now obviously I cannot just unplug the tonearm and have done with it...

I have just thought, I will put a dummy load on the phono inputs.

Just like to say, I have an electronics degree, but I am no whizz kid by any means, I have blown up so much stuff in my quest for good sounds. I have some knowledge though, and if I can help people, I will.

Thank you if you have taken the time to read this post. I look forward to hering and replying to comments, questions, and suggestions.
(2.) sounds like an acoustic feedback problem rather than something electronic (which would have to be magnetic in nature and quite strong to boot). This might be either directly via air or via vibration of the underlying surface (possibly with some help from room modes), in which case you would be well-advised to investigate better decoupling of your plinth (maybe the coupled resolator approach - plinth, flexible coupling, big heavy platter, flexible coupling, underlying surface), a more stable surface and optimized turntable vs. speaker placement. Room modes are also worth tackling. If you feel like throwing out the baby with the bathwater, you could even build an anechoic chamber for the TT.

Re: (1.), when do said charging noises occur? When turning on? Then investigate soft start circuitry. If they are occurring randomly, however, you may have stumbled across a typical problem of bigger toroidals, which is generating hum upon DC imbalance on the power line. Primaries' DC resistance can be so low for these that even a small amount of DC voltage can generate significant magnetic flux, pushing the transformer closer to saturation.

It also sounds like you should investigate protection circuitry.
Thank you for your reply. Yes you are right with the soft start, I forgot to mention I have soft start circuits on all transformers for the current supplies. I have speaker protection circuits which are built but not yet employed.

As for the charging noise, I guess it is the inrush current causing it, but it repetes every few minutes, not as strong as the start up hum/noise. I like the sound of the exsplanation you mention with regard to magnetic flux pushing the transformer near saturation. I am thinking the power supply caps may be a little too large, although the transformers are not even getting warm.

I did place a dummy input load which I set to 10 ohms as the cartridge resistance is 12 ohm. I discovered only one channel is getting the hum/noise. I can now compare the two channels.

I will onvestigate and compile the speaker protection power supplies.

I don't think the turntable is suffering feedback, but I apreciate your thinking.
Just a further explanation of what I meant by "power capacitor charging noise" I mean, when I turn the amp, the soft start allows a lower current to pass to the transformer, which in turn allows the DC ripple noise to be heard in the speakers and over several seconds is diminishes, but every so often, the DC ripple noise comes back all be it quieter, but it is still audible. I bought the softstart circuits from ebay and I don't have the specifications to hand. I recall thinking they are more than capable of delivering the power draw of my transformers. This is why i am thinking 100,000uf caps are a little too large for the supply.
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