Repairing a Musical Fidelity A1X

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#22
mz543578854 said:
I have played with it in LTSpice as well while working on it.
Since the A1(X) only has one NPN/PNP pair per channel, matching of pairs is not possible. Maybe you mean positive and negative to have as little DC as possible? I actually had planned to measure the DC and forgot 😆.
Will do the next days.

But I will not open the beast and reapply the thermal grease again, unless the offset is really high.
Click to expand...
No, not matching output devices. More like adjustments on resistors in case one channel was running much differently in bias mA than the other.
 
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#23
mz543578854 said:
In my sim I had about 800mA bias with 3.8Meg resistors. But probably can be caused by different models.
Click to expand...
745mA with 1.5 Meg in mine. 3.8Meg squeezes it down to 302mA. Could get 585mA with 3.8Meg if I replaced my 0.47Ω emitter resistors with 0.22Ω. As I saw two red stripes on your emitter resistors photos. 2.7Meg & 0.22Ω would do 812mA. But the MJ21195/21196 sim models I used wouldn't know peculiarities of the amp's circuit editions and its actual type semis in the little inferno box. 😉
 
#24
mz543578854 said:
Yeah, I saw in my simulations, that the amp is not ideal for 4 Ohm speakers. I am using it with 90dB 4Ohm speakers at the moment and it can play pretty load and still sounds fine to my ears.
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90dB spkr sens definitely helps. A friend had the A1 for years. I had listened to it with various speakers. Not much resolution by modern amp standards but enough to get the message across. With a particularly pleasant tone in a British signature way. Looks were unique and iconic too. 2023 reissue in a wider box as redesigned by MF and made in Taiwan is better done. Has more PSU reserve, better layout, -10dB line direct switch, a volume remote. Not too badly priced either considering things classic audio tend to be much overpriced today.
 
#26
@Salas, I played with various BJT models (which are used in schematics flowing around the net for the A1) in all stages of the amp. Bias varies only a little. Here are the end BJT models I originally used. But as I said, also with MJ21195/MJ21196 bias is comparable.

.MODEL 2N3055 npn
+IS=2.37426e-14 BF=129.119 NF=0.85 VAF=31.1252
+IKF=0.990922 ISE=2.47498e-10 NE=1.89002 BR=1.01252
+NR=0.924456 VAR=254.624 IKR=2.70227 ISC=2.47498e-10
+NC=2.90624 RB=3.66609 IRB=0.1 RBM=0.1
+RE=0.000352673 RC=0.0764459 XTB=1.34801 XTI=1.07207
+EG=1.206 CJE=9.03089e-08 VJE=0.513954 MJE=0.59999
+TF=1e-08 XTF=1.36696 VTF=1.02605 ITF=0.987296
+CJC=5e-10 VJC=0.400243 MJC=0.410238 XCJC=0.803124
+FC=0.661216 CJS=0 VJS=0.75 MJS=0.5
+TR=1e-07 PTF=0 KF=0 AF=1
Click to expand...

and
.MODEL MJ2955 pnp
+IS=2.72053e-14 BF=97.7866 NF=0.85 VAF=46.4019
+IKF=2.06624 ISE=1e-16 NE=2.89167 BR=1.32254
+NR=0.834451 VAR=265.121 IKR=2.91618 ISC=1e-16
+NC=2.94268 RB=5.18255 IRB=0.1 RBM=0.1
+RE=0.00048997 RC=0.090446 XTB=1.12701 XTI=1
+EG=1.05 CJE=9.24763e-08 VJE=0.50165 MJE=0.620554
+TF=1e-08 XTF=1.36685 VTF=1.02571 ITF=0.986243
+CJC=5e-10 VJC=0.400266 MJC=0.410181 XCJC=0.803124
+FC=0.671228 CJS=0 VJS=0.75 MJS=0.5
+TR=1e-07 PTF=0 KF=0 AF=1
Click to expand...
 
#31
You did a very good restoration of a deteriorated and badly tweaked unit. Along with Mark Hennessy's line/volume alternative you included on a nice custom pcb it should be a quieter & more reliable, more pleasing than stock, little warm tone amp to use for many years. Congratulations for your work and presentation. :up:
 
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