13-09-2017, 06:29 AM
(This post was last modified: 13-09-2017, 06:36 AM by Diabolical Artificer.)
Bit of an update video here - https://youtu.be/qHmO9qxaRNo I'm still slogging away at it, winding another mains transformer. I've found a cheap source of big toroid's on ebay, think it was £30 for a 700VA, which is a tad bigger than the other one I wound, IE 625VA. I went over the playing field yesterday to measure out the HT winding's, which are 2 x 243.3ft, but ran out of copper wire. I have some on order from Brocott's. Then it's a case of twisting the two together to form one bi-filar wire, bung it on a foot long "bobbin" then wind it on. With that done the other secs are a doodle.
I've also built a VU meter driver. It uses a TL074 opamp, one as precision rectifier, another as a voltage amp and another as buffer. I didn't use the last bit. The only modification I did was add a 100k preset on the IP, as the circuit as is, loaded the OP off the ECC83 too much. I've also had to add an attenuator SW because with an OP off the ECC83 of 6v RMS, this means the meter tops out , IE FSD, at about 35W. this is just a 6K8 R, with a SPST SW.
I also added a peak/clipping indicator, which turns an LED on at 5.9v RMS, glowing brightly at 6v RMS.
Lastly I've built a protection circuit that fires a triac at around 100mA cathode current. Each EL34 has a wire running off the cathode to the protection PCB. This is just a peak detector in essence. It took a lot of experimentation to get it right. As I said at 1v the triac fires, which supplies 12v to a relay, with cuts the negative mains to the mains tfmr. There is also an ultra bright diode on the front panel to indicate a fault.
These circuits are all on one PCB, with the PSU, which is a half wave rectified +/- 15v, stabilised by two 5W zeners.
If anyone's interested, I'll attach the circuits.
I forgot to say I have been using the amp to listen to music and it sounds well. However it's a bit bass heavy, which is easily solved. It's tricky to appreciate any difference though, to other amps, as it's a mono amp at present, so I stuck to old Beatle's stuff, a bit of Donovan before he went all trippy and Charlie Mingus sounds especially good as does Pentangle - double bass heaven.
Lastly for a while, I had to reduce NFB a tad, increasing the NFB R from 4 to 5k. This means NFB is about 1/50th. I'm still not completely au fait with the math's IE. closed loop gain etc and have no idea how to apply the result to reduce oscillation. I intend to build a simple circuit using an opamp to drive a single ended EL84 in an effort to nail the math's etc at some point soon.
Finally after all the changes THD is 0.04% @ 1/2W, 0.08% @ 12.5W, and 0.18 @ 50 and 112W using 1khz. F response is flat from 50hz to 6khz where it starts to roll off, down by 3dB @ 20khz. Square wave looks good at 1khz with a bit of rounding of the leading edges, but at 10khz it looks more like Grandad's blunt old #3 saw used for cutting breeze blocks. Meh, it'll d
The peripheral circuit's have been the most interesting and fun bit on this amp, I've learned a lot building them. I nicked most of them, but modified them somewhat, spending hours bread boarding circuits, good craic. I'll do a complete schematic when i get a mo and post.
Andy.
I've also built a VU meter driver. It uses a TL074 opamp, one as precision rectifier, another as a voltage amp and another as buffer. I didn't use the last bit. The only modification I did was add a 100k preset on the IP, as the circuit as is, loaded the OP off the ECC83 too much. I've also had to add an attenuator SW because with an OP off the ECC83 of 6v RMS, this means the meter tops out , IE FSD, at about 35W. this is just a 6K8 R, with a SPST SW.
I also added a peak/clipping indicator, which turns an LED on at 5.9v RMS, glowing brightly at 6v RMS.
Lastly I've built a protection circuit that fires a triac at around 100mA cathode current. Each EL34 has a wire running off the cathode to the protection PCB. This is just a peak detector in essence. It took a lot of experimentation to get it right. As I said at 1v the triac fires, which supplies 12v to a relay, with cuts the negative mains to the mains tfmr. There is also an ultra bright diode on the front panel to indicate a fault.
These circuits are all on one PCB, with the PSU, which is a half wave rectified +/- 15v, stabilised by two 5W zeners.
If anyone's interested, I'll attach the circuits.
I forgot to say I have been using the amp to listen to music and it sounds well. However it's a bit bass heavy, which is easily solved. It's tricky to appreciate any difference though, to other amps, as it's a mono amp at present, so I stuck to old Beatle's stuff, a bit of Donovan before he went all trippy and Charlie Mingus sounds especially good as does Pentangle - double bass heaven.
Lastly for a while, I had to reduce NFB a tad, increasing the NFB R from 4 to 5k. This means NFB is about 1/50th. I'm still not completely au fait with the math's IE. closed loop gain etc and have no idea how to apply the result to reduce oscillation. I intend to build a simple circuit using an opamp to drive a single ended EL84 in an effort to nail the math's etc at some point soon.
Finally after all the changes THD is 0.04% @ 1/2W, 0.08% @ 12.5W, and 0.18 @ 50 and 112W using 1khz. F response is flat from 50hz to 6khz where it starts to roll off, down by 3dB @ 20khz. Square wave looks good at 1khz with a bit of rounding of the leading edges, but at 10khz it looks more like Grandad's blunt old #3 saw used for cutting breeze blocks. Meh, it'll d
The peripheral circuit's have been the most interesting and fun bit on this amp, I've learned a lot building them. I nicked most of them, but modified them somewhat, spending hours bread boarding circuits, good craic. I'll do a complete schematic when i get a mo and post.
Andy.
