Stereo 70

[Slartibartfast]

Can the ST-70 run without a feedback loop?

Why would you want to?

Changing the feedback will change the amount of gain the amplifier has. I personally believe that amplifiers do not sound as good without some feedback.

The Van alstine modification changes the amplifiers response. It had ultra high response before and it was way above what was needed thus taxing the amplifier. After the modification the amplifier lives an easier life. Thus no need to change or eliminate the amount of feedback in my opinion.

Speaking of feedback in the ST-70, does Van Alstine move the feedback to a more common secondary tap? There are not many people using 16 ohm speaker systems these days.

[EWBrown]

For 8 ohm NFB, just multiply the original (16 ohms output) NFB resistor by 0.707, and the cap by 1.414 (sqrt of 0.5 and 2, respectively, as impedance is a "square" rather than a "linear" function. The closest standard resistor values are good enough, this doesn't require "heart surgery" levels of precision.

For 4 ohms, halve the original NFB resistor and double the cap value.

/ed B

[Slartibartfast]

For 8 ohm NFB, just multiply the original (16 ohms output) NFB resistor by 0.707, and the cap by 1.414 (sqrt of 0.5 and 2, respectively, as impedance is a "square" rather than a "linear" function. The closest standard resistor values are good enough, this doesn't require "heart surgery" levels of precision.

For 4 ohms, halve the original NFB resistor and double the cap value.

/ed B

Thanks Ed. That works for the B&O speakers upstairs. Now what is the math for an 8 ohm tap? I am thinking of the LaScalas... Yellow_Light_Colorz_PDT_09

[burnedfingers]

What is this going to gain you buy taking the feedback off the 8 or 4 ohm tap?

[Slartibartfast]

What is this going to gain you buy taking the feedback off the 8 or 4 ohm tap?

Well should not the feedback come from the impedance tap that is being used? Then this mod would give the proper level of feedback for the given impedance?


This sure seems to be a fairly common mod over on the dynaco forum.

[EWBrown]

It is best to take the NFB from the secondary tap that is actually connected to the speakers, though it can also be taken from the higher output impedance tap, as was done with the original ST35, ST-70, etc.

Taking NFB from the tap which is connectd to the speakers allows for the NFB to work with the speaker's dynamics, as well as those of the OPT.


Using the ST-35 for an example (only because I remember the R and C values), the standard 16 ohms NFB is 27K and 27 pF.

For 8 ohms, the resistor should be 19.2K (20K is good) and the cap, 39 pF.

For 4 ohms, the resistor would be 13.5K (13K is good) and 54 pF (56 pF is good).


I'll go look up the ST70 and run a similar comparison.

/ed B

[EWBrown]

For the original (7199) ST-70, the NFB resistor is 1K, connected to 16 ohms, and there is no parallel capacitor.

(update, I used a calculator this time, rather than rounded off mental math) Yellow_Light_Colorz_PDT_09 :o Yellow_Light_Colorz_PDT_11


For the 8 ohm tap, the resistor would be 680 ohms and for 4 ohms, 475 ohms (470 is OK).

If you want to make it freely adjustable, and add some extra dB of NFB range, use a 2K 10 turn trimpot in place of the 1K resistor.

The 390 pF mica feedback cap pfrom the lower UL tap should NOT be changed.

For the DIYtube ST70 driver board, the 16 ohm tap's NFB resistor is 2.7K and the cap is 220 pF.

For 8 ohms, the resistor should be 1.88K (1.8K or 2K is OK) and the cap, 311 pF. (300 pF or 330 pF is OK)

For 4 ohms, the resistor should be 1.3K, and the cap 440 pF (430 or 470 pF is OK).

The resistors can be 1% tolerance, and then closer to ideal resistance values can be chosen. I generally use 500V, 5% silver mica "kidney bean" caps.

Similarly, to make it more freely adjustable, use a 2K trimpot with a 1K ohm resistor in series. The caps will still have to be re-calculated for any "odd" NFB resistor values

HTH

/ed B

[GrooveGrinder]

All high quality amplifiers sound alike unless they are over-driven to the point that they produce distortion.

Why then don't "low quality" also sound alike??
Rather than, for instance, just not lasting as long.

What exactly does "high quality" mean?

[TomMcNally]

All high quality amplifiers sound alike unless they are over-driven to the point that they produce distortion.

Why then don't "low quality" also sound alike??
Rather than, for instance, just not lasting as long.

What exactly does "high quality" mean?

I think the point is this ... an amplifier is supposed to just
amplify ... NOT color the sound. A good amplifier just
adds gain, not flavor. A design that adds distortion at
various stages changes the sound - that's not always
"bad" but it's not "high quality"

There was a company back in the 80's that made broadcast
amps ... called "Straight Wire Audio" - you got out exactly
what you put in - just louder. Like wire with gain.