Ampdog
R.I.P. 23 June 2022
I have an 'Antique Audio Lab' "Monsoon" monoblock on my table for checking. Four X KT88 output etc.
Being naturally inquisitive, I took the cover off the output transformer (OPT) to see what gives inside this time. Nice looking; winding former not full, thicker wire gauge could thus have been used (lower losses). Two halves side-by-side for equal dc winding resistances for the two primary sides (like the original Williamson).
Then I measured the main statistics: Inductance at 5V 50Hz: 100Hy. Leakage reactance at 1kHz: 1,9mHy.
Hey? I checked the frequency response under the operating conditions into a 15 ohm resistive load. Output -3dB points at 8Hz and 460kHz!! A slight but sharp peak at 550kHz. Response of the whole amplifier: 12Hz - 150kHz ... without any NFB. (The amplifier is a zero NFB job.)
I am now, after a slight change in the circuit, adding a modest amount of NFB to get the quoted 3% distortion down. I have quite a problem to get rid of instability at 550kHz above about 18dB NFB! And this is a valve amplifier.....
This must be the best OPT that I have ever encountered - and that includes some. It is an about 150W job, UL, E-I core.
Apart from this, has anybody experience of this amplifier?
Being naturally inquisitive, I took the cover off the output transformer (OPT) to see what gives inside this time. Nice looking; winding former not full, thicker wire gauge could thus have been used (lower losses). Two halves side-by-side for equal dc winding resistances for the two primary sides (like the original Williamson).
Then I measured the main statistics: Inductance at 5V 50Hz: 100Hy. Leakage reactance at 1kHz: 1,9mHy.
Hey? I checked the frequency response under the operating conditions into a 15 ohm resistive load. Output -3dB points at 8Hz and 460kHz!! A slight but sharp peak at 550kHz. Response of the whole amplifier: 12Hz - 150kHz ... without any NFB. (The amplifier is a zero NFB job.)
I am now, after a slight change in the circuit, adding a modest amount of NFB to get the quoted 3% distortion down. I have quite a problem to get rid of instability at 550kHz above about 18dB NFB! And this is a valve amplifier.....
This must be the best OPT that I have ever encountered - and that includes some. It is an about 150W job, UL, E-I core.
Apart from this, has anybody experience of this amplifier?
