abbey road d enfer said:68k is too much for R7, impairs S/N. Divide by 10.
Get rid of R7; it loads the 1st stage so alters gain. Adjust C2 till you're satisfied.
You need a grid leak resistor (already told you) 1 Meg.
Why don't you implement Moamps' excellent suggestion of applying P48 to the center tap of the input xfmr via a single 3.4k?
abbey road d enfer said:68k is too much for R7, impairs S/N. Divide by 10.
Get rid of R7; it loads the 1st stage so alters gain. Adjust C2 till you're satisfied.
You need a grid leak resistor (already told you) 1 Meg.
Why don't you implement suggestion of applying P48 to the center tap of the input xfmr via a single 3.4k?
With the low cost of the chokes you can use 2 in series. This has 3 things going for it.
1st, the inductance is double so you get 1 more octave on the bottom end of the frequency range.
2nd, because the capacitance of the chokes acts as 2 caps in series you will get better high frequency response. In the preamp I built the 3dB down point went from somewhere near 30K to out past 70K. The single choke had a 4dB dip around 40K. With the pair of chokes the dip was only 2dB. There is a broad very low Q resonance around 60K so with 2 chokes in series it drops 2dB down near 30K, then rises back up gently and finally drops below the -3dB point somewhere above 70K.
3rd, you can mount the chokes on standoffs, bottom to bottom and wire the chokes with one winding backwards. This makes the pair of chokes hum buckers. In my test the pair of chokes picked up 20dB less hum than a single choke.
No 68K, we meant stick the 1M back in.
The other R7 isn't needed either, right, C2 is the thing. VR1 IS R7 already, that's the value you calculate against.
Listen/measure LS-10 sec ground return connected at V1 cathode instead of ground, likewise with LS-50 Pri ground to V2 cathode. Listen to C3 at the ground side of LS-50 primary instead of plate side. Another set of tricks from the ancients that's fallen out of style.
LS-10 sec and LS-50 pri: reverse each of those orientations and measure treble response. One side will be better than the other usually, sometimes dramatically. Relates to the orientation of winding capacitance to ground. Phase to the external world as needed after determination.
LS-10: if you're super tricky with the tightness of your layout you could consider a series parallel switch on the secondary windings to reduce gain at times when you have a hot mic and a slightly overloading front end, but the pad is too much of a gain hit. If you at all make that of any wiring length you will hurt treble response and invite noise intrusion, it'd have to be super convenient or clever in layout. I would not expect using only one secondary winding to be successful, as leaving one floating tends to create resonance effects that bodge up response.
Put a polarity switch on the output side rather than input side. Consider phase of your DI input relative to transformer input, it may be reversed from the best orientation of the transformer secondary. You may have to label your polarity switch as "Positive Polarity Mic / DI".
350VDC is a hell of a lot of B+ for a preamp. That's 25V higher than the majority of American 10W monitor amps. Most preamps topped out around 280VDC. And. You don't have a bit of filter capacitance, you want at least something between R5/6. 1mfd. 10mfd. 22mfd. 47mfd.
Choke feed, more output gain. Look at RCA OP-6 output. Hammond 156C, about $18. Buy two of them. If you pursue that you'll have to size C3 based on measurements, as there's LC resonance to account for, it can be your friend, it can be your enemy.
emrr said:HPF shouldn't see a changing rolloff with VR1 rotation. That isn't adding up to me. What freq range?
emrr said:C1 is still 10x too large.
This shouldn't be; are you sure you have connected the pot correctly? Or have you kept a grid leak resistor?CurtZHP said:The "louder" you go with the HPF switched in, the higher the roll-off point.
abbey road d enfer said:This shouldn't be; are you sure you have connected the pot correctly? Or have you kept a grid leak resistor?
That is correct.CurtZHP said:I've seen a lot of designs that use a 0.1uF cap in that position. Wouldn't that affect low-end response in some way? I've understood that the lower a cap's value, the higher the frequency it passes. I assumed that meant that the lower the value, the less low frequencies pass.
That is both wrong AND right! It depends on the load. Here, with 250kohm load, a 0.1uF introduces a -3dB LF cut-off at 6.3Hz.I had read somewhere (probably to do with guitar amps...) that a 0.1uF cap doesn't pass 100% until you get to 3kHz!
That would imply a load of about 100kohm...Funny thing is, I've run across a coupling cap calculator for guitar amps that shows a 0.1uF cap passing everything down to about 18Hz!
I have seen that it's gone from this schemo, but the actual realization is not always 100% in conformity with plans.CurtZHP said:See my Heavy_ver5 schematic.
emrr said:LS-10: if you're super tricky with the tightness of your layout you could consider a series parallel switch on the secondary windings to reduce gain at times when you have a hot mic and a slightly overloading front end, but the pad is too much of a gain hit. If you at all make that of any wiring length you will hurt treble response and invite noise intrusion, it'd have to be super convenient or clever in layout. I would not expect using only one secondary winding to be successful, as leaving one floating tends to create resonance effects that bodge up response.
emrr said:yep. that and the instrument jack have to be a close and short lead as possible.
emrr said:I don't see a question?
emrr said:Ah. Choke replaces the plate B+ feed resistor.
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