23-02-2017, 05:55 AM
Doodlebug - I should of said there's a kid inside that likes the thought of big amps, I'm responsible on the whole though my neighbors where treated to The Russian's are coming by Val Bennet at louder than normal volume briefly on Sunday. I had to point the speaker out the door as the windows vibrate alarmingly if not.
My main speakers are 86dBA SPL @ 1m 1w pink noise which work well with a transistor amplifier which is 70w. At about half volume the bass moves your diaphram 8 feet away, it's only kicking out about 1w, I measured it and was very surprised. What I don't get Mark in your post above is how the volume should be set. To get full power ish your amplifier would need to be at 3/4 volume say, so that would work if your listening to a record with a large dynamic range, a bit of classical say where you have quiet passages then a crescendo at the end. For rock music which is full on from start to finish though it would be uncomfortable. A better set up would be with a separate pre-amp and power amp. Preamp set low, power amp high. Hope you get the gist.
Going back to the amplifier, I direct coupled the PS to the cathode follower's to keep the phase difference between IP and OP as low as poss.
I made a few changes yesterday and things are a lot better. I tried a 4700p cap and the result was terrible. Did some more calculations, did some looking online at at some schematic's I have and changed the two 150k grid R's for 47k and used 220n caps. That was a lot better. What made the big difference though was changing the way I got the negative bias to the two sides of the OP stage. Now things are a lot better.
The cathodes of the PS are fed by a transistor CCS which is powered by a separate PSU from the main HT PSU. It has a separate tfmr 50v, 78v DC once rectified. It's positive goes to ground giving - 78v DC, this feeds the negative bias for the OP stage too. Not sure if this will be classed as de-coupled. I'll draw a schematic; so far I havn't really looked at the PSU, it could do with improvement. I think the CCS could be improved too to make it higher impedance.
I'll rig a protection circuit up Trevor that will cut power to the amp in case of no bias or excessive OP stage current. To get max power fixed bias is the only way to go, though I did think of doing both; a bit of self bias and fixed, will see.
Andy.
My main speakers are 86dBA SPL @ 1m 1w pink noise which work well with a transistor amplifier which is 70w. At about half volume the bass moves your diaphram 8 feet away, it's only kicking out about 1w, I measured it and was very surprised. What I don't get Mark in your post above is how the volume should be set. To get full power ish your amplifier would need to be at 3/4 volume say, so that would work if your listening to a record with a large dynamic range, a bit of classical say where you have quiet passages then a crescendo at the end. For rock music which is full on from start to finish though it would be uncomfortable. A better set up would be with a separate pre-amp and power amp. Preamp set low, power amp high. Hope you get the gist.
Going back to the amplifier, I direct coupled the PS to the cathode follower's to keep the phase difference between IP and OP as low as poss.
I made a few changes yesterday and things are a lot better. I tried a 4700p cap and the result was terrible. Did some more calculations, did some looking online at at some schematic's I have and changed the two 150k grid R's for 47k and used 220n caps. That was a lot better. What made the big difference though was changing the way I got the negative bias to the two sides of the OP stage. Now things are a lot better.
The cathodes of the PS are fed by a transistor CCS which is powered by a separate PSU from the main HT PSU. It has a separate tfmr 50v, 78v DC once rectified. It's positive goes to ground giving - 78v DC, this feeds the negative bias for the OP stage too. Not sure if this will be classed as de-coupled. I'll draw a schematic; so far I havn't really looked at the PSU, it could do with improvement. I think the CCS could be improved too to make it higher impedance.
I'll rig a protection circuit up Trevor that will cut power to the amp in case of no bias or excessive OP stage current. To get max power fixed bias is the only way to go, though I did think of doing both; a bit of self bias and fixed, will see.
Andy.
