2. amps-preamps
  3. 1095615957

Problem with Musical Fidelity A3cr pre-amp

Has anyone had this problem?

I have a 3.5 year old A3cr preamp that distorts on loud transients in high frequencies and on sustained pure high frequency sounds. I tried everything else and I'm pretty sure its the pre-amp. It depends on volume and frequency make-up of source, but it is quite annoying. It is not equally bad on all inputs, but it is there.

I use the pre-amp with an A3cr amp, analysis plus cables and Magnepan 1.5Qr's. I tried different inputs, cables, amp and speakers, the problem really seems to be the pre-amp. I originally thought it was the speakers since it sounds a little like a torn cone on a paper tweeter, but it is the pre-amp.

Does this sound like anything you've heard of? What can be done? I live in the Boston area, but I have the original boxes if I need to ship it. Does anyone have any idea of cost for repairs?

Thanks
fineberg

Showing 1 response by timpani

timpani
40 posts
 
The problem is in the first gain stage and is due to slew limiting in the feedback network. I resolved the problem by replacing 2 x 47pf capacitors (1 per channel) with 22pf. This reduces the amount of local shunt feedback and removes the slewing problems. The 47pf caps are ceramics and (from memory) they are the only 47pf caps on the board so they will be easy to find. I also replaced them with polystyrenes for better performance. After the mods I could hear no ill effects and in fact I felt that if anything there was an overall improvement as well as better transient performance. I noticed the problem more on SACD, where the extra bandwidth was further compromised by the slew limiting. BTW, I was clearly able to see the problem on a CRO with a 10KHz square wave applied. It was evidenced by a massive overshoot on the leading edge. After the mod, the 10KHz waveform was pure. I also submitted my findings and corrections to MF. Unfortunately, they never contacted me or thanked me.

I hope this helps.
More to discover