Hi,
everything begun few days ago, when I measured the slew rate of my new borne Aleph 30, finding it was just 4 or 5 V/us; being worried about transient intermodulation distortion (that is said to be related to low SR), I tried to get it higher.
I suspected that the guilty was the mosfets input capacitance in the amplifier section (I used IRFP350, cheaper but with higher input capacitance than IRFP240), so, just for test, I put an emitter follower between the differential stage and the gates of power mosfets; I used a BJT (BD139), with a trimmer to set its bias current.
At low bias current (less than 10 mA), the slew rate was the same, but when I increased the bias current to 15-20 mA, the SR increased to about 11 V/us; for higher current, no further improvement.
After that, I measured again the amplifier distortion, finding it was 6 to 10 dB lower than before, also at low frequency.
Some values I got:
F= 999 Hz, Vout = 14 Vrms (Pout = 24.5W/8 ohm) THD= -61 dB
(second harmonic: -63 dB, third harmonic: < -70 dB)
F= 6301 Hz, Vout = 14 Vrms (Pout = 24.5W/8 ohm) THD= -54 dB
(second harmonic: -64 dB , third harmonic: -56 dB)
Two tones IM:
F1= 11 KHz, Vout = 7 Vrms; F2= 12 KHz, Vout= 7 Vrms (total peak amplitude 20 Vpk)
1KHz IM product: - 68 dB (below each tone amplitude)
10KHz IM product: - 70 dB (below each tone amplitude)
13KHz IM product: - 65 dB (below each tone amplitude)
I’m happy of this, but now, before starting new improvements, (as using a mosfet source follower, instead that BJT emitter follower), I’m wondering if lowering distortion can cause the loss of Aleph typical sound, making it cleaner but less pleasant.
I can’t do a listening test (at the moment I don’t have any good speaker), so the question is:
- is there anybody (more expert in Aleph’s sound than me) that can tell me how much is correct to reduce distortion without loosing the quality of sound?
Thanks
Fulvio
everything begun few days ago, when I measured the slew rate of my new borne Aleph 30, finding it was just 4 or 5 V/us; being worried about transient intermodulation distortion (that is said to be related to low SR), I tried to get it higher.
I suspected that the guilty was the mosfets input capacitance in the amplifier section (I used IRFP350, cheaper but with higher input capacitance than IRFP240), so, just for test, I put an emitter follower between the differential stage and the gates of power mosfets; I used a BJT (BD139), with a trimmer to set its bias current.
At low bias current (less than 10 mA), the slew rate was the same, but when I increased the bias current to 15-20 mA, the SR increased to about 11 V/us; for higher current, no further improvement.
After that, I measured again the amplifier distortion, finding it was 6 to 10 dB lower than before, also at low frequency.
Some values I got:
F= 999 Hz, Vout = 14 Vrms (Pout = 24.5W/8 ohm) THD= -61 dB
(second harmonic: -63 dB, third harmonic: < -70 dB)
F= 6301 Hz, Vout = 14 Vrms (Pout = 24.5W/8 ohm) THD= -54 dB
(second harmonic: -64 dB , third harmonic: -56 dB)
Two tones IM:
F1= 11 KHz, Vout = 7 Vrms; F2= 12 KHz, Vout= 7 Vrms (total peak amplitude 20 Vpk)
1KHz IM product: - 68 dB (below each tone amplitude)
10KHz IM product: - 70 dB (below each tone amplitude)
13KHz IM product: - 65 dB (below each tone amplitude)
I’m happy of this, but now, before starting new improvements, (as using a mosfet source follower, instead that BJT emitter follower), I’m wondering if lowering distortion can cause the loss of Aleph typical sound, making it cleaner but less pleasant.
I can’t do a listening test (at the moment I don’t have any good speaker), so the question is:
- is there anybody (more expert in Aleph’s sound than me) that can tell me how much is correct to reduce distortion without loosing the quality of sound?
Thanks
Fulvio
NB
it's not just matter of follower , it's also matter of greater NFB , tnx to increased OLG
to introduce any follower , one needs to increase drain resistor in LTP , to compensate for Vbe ( in case of bip follower ) or Vgs ( in case of mosfet follower)
result is increased gain of input LTP , that's why OLG is increased too
it's not just matter of follower , it's also matter of greater NFB , tnx to increased OLG
to introduce any follower , one needs to increase drain resistor in LTP , to compensate for Vbe ( in case of bip follower ) or Vgs ( in case of mosfet follower)
result is increased gain of input LTP , that's why OLG is increased too
No, I did'nt increase the drain resistor; the differential pair works now slightly unbalanced, that is 11.5 mA in the first mosfet and 8.5 mA in the second; the 1.5 mA current increase in the first mosfet compensate the 0.6 V of BJT vbe.
An increase of 3 mA of the current generator should be right to re-balance the differential.
Fulvio
An increase of 3 mA of the current generator should be right to re-balance the differential.
Fulvio
I would not use a follower - I think it would alter the tonal balance of the amplifier. If I were you, I would simply increase the tail current of the input differential. Then you have to change (lower) two resistor values - one that is defining the tail current - the other the drain resistance of the input differential. (Maybe it would be necessary to mount a heat sink on the current generator for the input differential). If you double the current, the Slew rate should double.
Thanks Knutn, I’ll try for sure what you suggest, even if I don’t like to increase signal mosfets dissipation, because they already works in a hot ambient; I built my Aleph as two monoblocks with and a separate power supply; each monoblock is made up with two heatsinks; in the space (30 mm) between their internal sides is located the PCB; so everything inside goes near the heatsink temperature (60 – 70°C); for this reason, the signal mosfets have already a little heatsink that can’t be enlarged.
Fulvio
Fulvio
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