SOLDANO SLO preamp analysis

[Lynxtrap]

I'm not even sure which gain stage you are focusing on here.
I thought this was about the 3rd stage, the so called "cold clipper", V2B? If so, I'm looking forward to further results.

Do you have an actual amp to put a 'scope in, or is it all math?

 

[Pete Farrington]

Any idea where they got the formula in RDH4?

This is the thread where that calculator was introduced (post 27) / discussed / reviewed

Choosing cathode bypass values - Music Electronics Forum

music-electronics-forum.com

 

[ltournell]

Here is my chicken scratch. I haven't typed it up yet.

View attachment 1113601

Your formula seems accurate

 

[ltournell]

I'm not even sure which gain stage you are focusing on here.
I thought this was about the 3rd stage, the so called "cold clipper", V2B? If so, I'm looking forward to further results.

Do you have an actual amp to put a 'scope in, or is it all math?

We get lost in all those digressions. The 3 rd stage was talked about in another thread and this one was supposed to synthesize and be more clear ! . As I said in the top post , the preamp is simulated with LTSpice and the computations made from load lines . It would be difficult to get gm, rak , the input/output impedances etc.. with a scope anyway. And I haven't got a spectrum analyser .

 

[ltournell]

(continued)

If we resume where we left in the top post here is a table , just for info, of the various levels , which show that the stages are early overdriven :
It reads like this :

There is a frame of 6 lines for each position of the gain pot ( from 0.5 to 7)
Each line gives the entry level of the matching string
2 nd column : the level in Volts peak for peak of the entry of the pot
3 rd column : gain in percentage (according to the position of the pot)
4 th column : the level at the slider of the pot (entry to stage 2 = first red arrow)
5 th column : the output of stage 2 into stage 3
6 th column : the output of stage 3 into stage 4
7 th colum : the output of stage 4

All levels are theoretical sine - and all figures in red show overdriven levels ( with 74 kV out of stage 4 with the pot gain to 7 !)

For convenience I repeat below the schematic of post 1 :

To be continued with the next step : identification of the harmonics ...

 

[ltournell]

Following our discussion "Designing etc..." I have dropped the term cut-off frequency for the -3 dB limit under the upper shelf of the gain curve and adopted Pete's suggestion : upper transition limit . I have also adopted the Ten Over's half-boost frequency formula which proves very accurate considering the RDH4 demo schematic p 484 redrawn here :

Magnitudes :

Phases :

Ten over's formula here :

exactly matches the values shown on the phases diagram knowing that the half-boost frequency occurs at the crest of each curve when the phase variation sign changes to null before turning negative.

So if we recall the SOLDANO preamp schematic here :

a more complete data table can be shown like this :

Now varying the only gain pot of the preamp we can notice that the weakest signal will easily overdrive at least the output as shown on the table of my previous message where all red figures match overdriven inputs : do not learn this table by heart ! . The six lines each time give the values of the attack of each open string of the guitar ( with 490R humbuckers) . The measurements are taken at the red arrow junctions on the schematic above. All values are Vpp and fictitious pure sine ! ( 74 kVpp at most ! ).

(to be continued ...)

 

[gmm52]

Summary of science: Soldano's sound great.