Nick's little project
- Thread starter NickK
- Start date
Little update in that there's not much - I've had to sort out other projects (including remortgaging, laying garden paths, family visits etc).
I have been looking at the wood work in the interim. The size initially is 705x478x275 which is perhaps too large for a 1x12. I've been looking at other cab sizes, for example some Zilla cabs (ignoring they're not combos) - 620x490x280, 620x470x360 and some Marshall cabs/combos 610x535x230, 510x490x245, 620x490x252 (combo), 500x406x245 (combo valve 10" speaker).
So I'm thinking that a possible reduction in width to 620mm, throw away the golden ratio and then simply look at making it a 620x490x278. The ply I have is 12mm, so I will double it up because I want to be able to sit on it (and it's not going anywhere on a regular basis).
I have been looking at the wood work in the interim. The size initially is 705x478x275 which is perhaps too large for a 1x12. I've been looking at other cab sizes, for example some Zilla cabs (ignoring they're not combos) - 620x490x280, 620x470x360 and some Marshall cabs/combos 610x535x230, 510x490x245, 620x490x252 (combo), 500x406x245 (combo valve 10" speaker).
So I'm thinking that a possible reduction in width to 620mm, throw away the golden ratio and then simply look at making it a 620x490x278. The ply I have is 12mm, so I will double it up because I want to be able to sit on it (and it's not going anywhere on a regular basis).
Well the Mrs' mum is pending a more detailed diagnosis and so she's intending to take a sabbatical to assist, so saving some pennies for the time being - the amp is not dead just will need to take a back burner 
I have the wood so I may continue with that in the interim to keep me busy.
I did an experiment on the A220 at the weekend - specifically looking at the power supply performance across the frequency range and the noise level. Basically I used 8R 100W dummy loads, put the signal generator (both outputs synchronised) and then used the scope Bode plot to run across a frequency response. Then I measured the response of both the pre-amp opamps and the power supply for the pre-amp opamp.
In the past I changed the caps used in that pre-amp power supply, upping the capacitance a little but also dropping the ESR to about 5mOhm according to the capacitor specs (low ESR in parallel). The result means the existing power supply provides a better power response but it's still using the same old technology (it has a regulator but it's a simple design so suffers from a few issues).
Two points - the opamp pre frequency response is good, the noise level is about 60-80dB which means the opamp power supply sucks. Measuring the power supply for the opamp, you can see the volume has an impact on the frequency response of the opamp - that is not desirable as it means the sounds varies depending on volume (irrespective of the class A/A2 transition). So in short that power supply response should be flat. It says that the power supply main power banks are too small for higher volumes and that even smaller volume changes cause a difference in frequency response driven by the power supply. Being flat means that the power supply should/needs to have a low impedance across the power supply range and the power should be as quiet as a mouse for the opamp on the front to perform to it's best.
In the past I've designed a very very low noise power supply for the ADC (in this thread), so the temptation is to take that design and apply that to the opamp itself. This means bypassing the old pre-power supply, replacing that by a regulated shunt power supply which then results in a very low impedance for power.. (ie it can get current at high speed and low sustained current with equal ease) and the power will be ultra low noise.
Todo that I'm tempted to replace the opamp and simply build a pre-amp opamp board (perhaps using a better singles/dual ICs) then building the power directly on that board with caps etc. I'd also add space for the decoupling film caps that are current floating and then simply use shielded wire to connect. Perhaps on a 100x100mm board so I can get it on the size for prototype PCB orders. If I do the pads correctly, I may be able to support a number of different opamps
That would keep me busy whilst not costing the earth (the tube amp would be quite expensive).
I have the wood so I may continue with that in the interim to keep me busy.I did an experiment on the A220 at the weekend - specifically looking at the power supply performance across the frequency range and the noise level. Basically I used 8R 100W dummy loads, put the signal generator (both outputs synchronised) and then used the scope Bode plot to run across a frequency response. Then I measured the response of both the pre-amp opamps and the power supply for the pre-amp opamp.
In the past I changed the caps used in that pre-amp power supply, upping the capacitance a little but also dropping the ESR to about 5mOhm according to the capacitor specs (low ESR in parallel). The result means the existing power supply provides a better power response but it's still using the same old technology (it has a regulator but it's a simple design so suffers from a few issues).
Two points - the opamp pre frequency response is good, the noise level is about 60-80dB which means the opamp power supply sucks. Measuring the power supply for the opamp, you can see the volume has an impact on the frequency response of the opamp - that is not desirable as it means the sounds varies depending on volume (irrespective of the class A/A2 transition). So in short that power supply response should be flat. It says that the power supply main power banks are too small for higher volumes and that even smaller volume changes cause a difference in frequency response driven by the power supply. Being flat means that the power supply should/needs to have a low impedance across the power supply range and the power should be as quiet as a mouse for the opamp on the front to perform to it's best.
In the past I've designed a very very low noise power supply for the ADC (in this thread), so the temptation is to take that design and apply that to the opamp itself. This means bypassing the old pre-power supply, replacing that by a regulated shunt power supply which then results in a very low impedance for power.. (ie it can get current at high speed and low sustained current with equal ease) and the power will be ultra low noise.
Todo that I'm tempted to replace the opamp and simply build a pre-amp opamp board (perhaps using a better singles/dual ICs) then building the power directly on that board with caps etc. I'd also add space for the decoupling film caps that are current floating and then simply use shielded wire to connect. Perhaps on a 100x100mm board so I can get it on the size for prototype PCB orders. If I do the pads correctly, I may be able to support a number of different opamps
That would keep me busy whilst not costing the earth (the tube amp would be quite expensive).
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Hi Nick, would you mind sharing a high-res version of your power supply? I'm building a circlotron at the moment and can't seem to crack the issue of a regulated floating power supply...Your amp IS your power supply:
The bottom left is the two amp channels.. the rest of the blocks are the regulated power supplies. Currently they're simulating in LTSpice but the small single model was showing a <1mA ripple on the 170V supply - about 16mV
With a 1V 1KHz signal input.. No 50/100Hz noise.. below 120dBV.. I suspect that will be rather quiet in real life too.
I’ll have to have a look tomorrow ans post some higher res images.
The main thing is the isolated power transformer so that each PSU floats.
Do you have a rough schematic of yours?
For low voltage, a normal regulator works but for higher voltage you need a mosfet in front of the regulator. That mosfet needs to have a full voltage DC safe-operating-area. a lot of the more modern mosfets are only able to switched pulsed and not DC.
Edit - I had a look and I don’t have the high res or LTSpice files unless they’re backed up.
Ping up the schematic and I can probably help debug.
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Well my old Harlechs and front Keep speaker have died.
All of the drivers except one have cracks, even holes/disintegrating, plus the keep tweeter has a crack too on closer inspection.
I've pull the drivers, tweeters and cross overs out. The bodies will go to the tip and I'll recycle what I can. The crossover ICW caps (10uF, 650V) are worth saving as they're now sold as clarity caps at £70/each! There's other components I can use in there too - I may make a power soak for the guitar amp out of the components (at least some I can reuse).
All of the drivers except one have cracks, even holes/disintegrating, plus the keep tweeter has a crack too on closer inspection.
I've pull the drivers, tweeters and cross overs out. The bodies will go to the tip and I'll recycle what I can. The crossover ICW caps (10uF, 650V) are worth saving as they're now sold as clarity caps at £70/each! There's other components I can use in there too - I may make a power soak for the guitar amp out of the components (at least some I can reuse).