Rerurn to Romy the Cat's Site


In the Forum: Melquiades Amplifier
In the Thread: Valve Preamplifier for Macondo/DSET
Post Subject: NicePosted by anthony on: 8/5/2020
 Romy the Cat wrote:
Hm you are true in the “terra incognita” territory as far as I concern. The preamps are mystery to me. All my attempts to make any decently sounding preamp or even buffer failed. To me Placette was truly God sent and if I got burned that I am looking with fear what will I do.
 


Early days of course, but at the moment the 10y preamp sounds nice.  I do like it.  There may be ways to wring more performance from it and I am mulling over several alternative options for loading the tube.  No doubt those DHT's do enable something nice with the sound, that hint of extra dynamics not really heard elsewhere.
 Romy the Cat wrote:
  The idea to make a tube-based preamp is noble one and 10Y is a very good tube. The 30R before the autoformer is good and if you are able to drive it lover with autoformer them it is wonderful, even I am not so comfortable with the ideas of autoformers. The changing preamp output impedance is very bad for the Milquades as each preamp output impedance will change the setting of the input filters. However, if your impedance slides under a few Ohms then it might have no auditable impact at all. You can calculate how your moving output impedance impacts your amp filters. I do not care about the fact that it moves a few Hz up or down. I am care about phase. If you time aligned two channels and move volume of your preamp, then will the changing the preamp output impedance result the channels juts out of alignment? They certainly will BUT the it is possible since the impedance number are so low the timing error will be very much within the margin of errors of your time alignment.
 
I do not have the answer if the changing of the Rout would be prohibitive in this situation. What I would like you to consider is that in 10 years you will find your DPoLS and then you might realize that your playback can maintain the DPoLS only with let sat 30% of your volume and then the phase micro-shifts kick the system out of DPoLS. I do not mean to “scare” you, juts present you an architectural concept.  

Romy the Cat


I am not sure how phase changes depending on the activated winding of the autoformer, unless you are saying that the preamp Zout will possibly change the DSET filter phase somehow?  

Below is an old DSET input impedance measurement.  I bumped the 10kHz Channel F filter aircaps upon reassembly and did not bother to reset them (better to get the amp into the room) so above 10kHz may not be indicative.  The green phase and yellow impedance lines deviate near each of the crossover frequencies but phase is pretty much well controlled until the highest octave or two, near the bad filter.  This measurement needs re-doing.

DSET Input Impedance 2V Pink Noise FFTsmaller.jpg


With the autoformer at the output of the preamp, impedance will vary from 30R to 1R in my normal listening volume range.  Of course I can swap it to the preamplifier input and have 30R full-time regardless of attenuation.



Rerurn to Romy the Cat's Site