Alright! Thanks for all the responses!
I took the NFB out of the circuit completely and paralleled the feedback and cathode resistors to ground to ensure proper bias and it definitely sounds different, but I surely need to clean up some of the distortion. I'm going to try it woithout nfb for a week and see what I like.
Also, I'm not sure whether I should have kept the 910p cap in parallel with the resistor bias. I didn't and I think I should have to ensure no local NFB.
So... cap in or out? And is the difference that I will have local NFB with the cap out and no NFB with the cap in or something else?
Kofi
I took the NFB out of the circuit completely and paralleled the feedback and cathode resistors to ground to ensure proper bias and it definitely sounds different, but I surely need to clean up some of the distortion. I'm going to try it woithout nfb for a week and see what I like.
Also, I'm not sure whether I should have kept the 910p cap in parallel with the resistor bias. I didn't and I think I should have to ensure no local NFB.
So... cap in or out? And is the difference that I will have local NFB with the cap out and no NFB with the cap in or something else?
Kofi
Hi Kofi,
No no no. We must be careful here about that 910pF capacitor bypassing the feedback resistors (5K1 and 4K7 in parallel). The effect of the cap between speaker terminal (which has a definite signal voltage across it, quite more that that over the cathode 830 ohm) and cathode, is not the same as when going simply from cathode to ground. The d.c. through the resistors will remain the same as the transformer output winding has negligible resistance, but not a.c. conditions with reference to the cap.
Anyway, this does not matter as calculation shows that the 910pF starts having an effect (the -3dB point) only at about 71 KHz. In other words it is very much there for feedback stability way above the audion region, not as a roll-off. Since there is no feedback when you take the feedback resistance to ground, it presence or absence is then insignificant.
Kofi, I myself will not be able to react to any possible reply of yours until next week because I am leaving on holiday, but I am sure others will oblige.
Good luck.
No no no. We must be careful here about that 910pF capacitor bypassing the feedback resistors (5K1 and 4K7 in parallel). The effect of the cap between speaker terminal (which has a definite signal voltage across it, quite more that that over the cathode 830 ohm) and cathode, is not the same as when going simply from cathode to ground. The d.c. through the resistors will remain the same as the transformer output winding has negligible resistance, but not a.c. conditions with reference to the cap.
Anyway, this does not matter as calculation shows that the 910pF starts having an effect (the -3dB point) only at about 71 KHz. In other words it is very much there for feedback stability way above the audion region, not as a roll-off. Since there is no feedback when you take the feedback resistance to ground, it presence or absence is then insignificant.
Kofi, I myself will not be able to react to any possible reply of yours until next week because I am leaving on holiday, but I am sure others will oblige.
Good luck.
Kofi Annan said:
Yes you will have local feedback without the cap. But you'll have it with the cap too. The cap is so small it doesn't make much difference. The corner frequency of a 910pf cap in parallel with 618R is around 285kHz. Doubt you'd hear the difference.
If you wanted to ensure no local feedback, you have to put something like 470uf in there. Of course, that would change your gain calculations, and sensitivity, increase distortion, etc..
In the schematic as drawn (910pf//2.45k), it starts to roll off the feedback at about 70kHz. It's not there to really tune the sound, only to prevent positive feedback from causing oscillation (which will sound bad).
Sheldon
edit: Johan beat me to the punch. If there is ever a discrepancy between my answer and his; use his.
Kofi,
Correction. If there is ever a discrepancy, examine both and question it if necessary. Thus you will learn and both Sheldon and myself also.
Sheldon,
Thanks for the compliment. I sometimes need it. (Looking at the timing of the previous posts, perhaps we should also question telepathy. But enough mysteries here - not that as well!)
Late reply because I was away.
Regards
Correction. If there is ever a discrepancy, examine both and question it if necessary. Thus you will learn and both Sheldon and myself also.
Sheldon,
Thanks for the compliment. I sometimes need it. (Looking at the timing of the previous posts, perhaps we should also question telepathy. But enough mysteries here - not that as well!)
Late reply because I was away.
Regards
OK-- I think I have found the combination that works for me.
I used an 8.4K resistor for the NFB and a 665R for the cathode to ground, which should give me around 620R for cathode bias. This also gives me plenty of gain and cleans up a lot of the unmentionables without killing the detail and sterilizing the music. I left the 910p bypass cap on the NFB resistor. By my calculations (which probably suck) it should roll off at just over 20K, which should not impact listening, right?
Wow. It sounds really nice. It's a totally different sound than SET, in my opinion. It's much more lively and detailed, but some of that could be due to the increased power. Also, lots more bass but again, it could be better OPTs that are helping in that area.
Overall, I'm still drawing my conclusions, but I really like it a lot. This has been a really satisfying project. Thanks again to all who helped my dumb *** out on this.
Owe you one.
Kofi
I used an 8.4K resistor for the NFB and a 665R for the cathode to ground, which should give me around 620R for cathode bias. This also gives me plenty of gain and cleans up a lot of the unmentionables without killing the detail and sterilizing the music. I left the 910p bypass cap on the NFB resistor. By my calculations (which probably suck) it should roll off at just over 20K, which should not impact listening, right?
Wow. It sounds really nice. It's a totally different sound than SET, in my opinion. It's much more lively and detailed, but some of that could be due to the increased power. Also, lots more bass but again, it could be better OPTs that are helping in that area.
Overall, I'm still drawing my conclusions, but I really like it a lot. This has been a really satisfying project. Thanks again to all who helped my dumb *** out on this.
Owe you one.
Kofi
Kofi Annan said:
Well, yes and no. The roll off will be just above 20k, but it may impact listening, as it will have an effect on phase in the audible range. You might hear a difference if you put a 470 or 330pf cap in there. That's not to say which you'd prefer, though.
Sheldon
Hmmm... That's interesting. I have noticed a bit of a warble in the higher frequencies. Could this be the phase effect of the 910p cap?
I'd like to try a 470 or 330, but could I just saunter down to the Rat Shack and pick up some ceramic disc caps for that, or should I order some sliver mica deals from Mouser and wait?
Kofi
NPO ceramics will do OK, but if you're near headquarters in New York, you can easily find film caps in that size. Silver micas are a slam dunk; you can run into any little electronics shop, and if your chauffeur gets a ticket for parking across that hospital ER entrance next door, no big whoop, you've got diplomatic immunity.
BTW... now that you will be leaving office soon... wouldn't you share a last black heli ride with the guys here to commemorate your success at Beauvois Valley ? 😀
Gastón
Gastón
I'll just order some micas. I won't have a chance to re-solder until this weekend anyway.
Also, I have thought about this and maybe I made a bad component choice. I have an Allen Bradley carbon comp as my 4.7K gridstopper, but I wonder if I shouldn't have gone with a metal film for this position.
I understand that the carbon comps help reduce "drift", but I wonder if I coundn't tolerate some to improve the sound.
Any advice on this one?
Yeah, well, normally I'd say there ain't no black helicopters, but now that I'm being "replaced", what the hell. No drinking or eating inside, though. I just had it pimped and detailed.
Kofi
Kofi Annan said:
I've no real experience here, so this is just based on book learnin. Drift in that position is not an issue, because the resistance value is not critical. It's only there to damp possible oscillation. Film resistors are quieter than carbon comps and drift less. However, that resistor is passing very little current, so noise shouldn't be an issue. My guess is that the difference will be very subtle, if you hear any. I wouldn't obsess over it.
Sheldon
Metal firm resistors are inductive (unless specifically built not to be so), thus its usage as grid stoppers is not advisable.
Gastón
Gastón
Good deal. Thanks.
So, I'm definitely hearing what I believe are "phase issues", such as the top end ride cymbal and high notes on the trumpet having some warble and washout. Is this what I'd be hearing given the cap value will roll the feedback off around 20K or so as I have it?
Kofi
So, I'm definitely hearing what I believe are "phase issues", such as the top end ride cymbal and high notes on the trumpet having some warble and washout. Is this what I'd be hearing given the cap value will roll the feedback off around 20K or so as I have it?
Kofi
Uhhhh... does Radio Shack have a Tandy Square Wave / Osciloscope combo for $9.99? If not, I'm afraid I'm out of luck.
Yikes.
Any other ideas?
Kofi
Yikes.
Any other ideas?
Kofi
Hey Kofi,
It's not a Tandy, but is in your price range 😀
If you don't score this one, keep looking (note the mis-spelling on this listing..probably why it's going so cheap).
As for the square wave gen., you can put one together for under 5 bucks..just search for a suitable schematic.
Once you have this combo, you will insist Mrs. Annan give you a couple extra lashings for trying to do without for so long.
-Casey
It's not a Tandy, but is in your price range 😀
If you don't score this one, keep looking (note the mis-spelling on this listing..probably why it's going so cheap).
As for the square wave gen., you can put one together for under 5 bucks..just search for a suitable schematic.
Once you have this combo, you will insist Mrs. Annan give you a couple extra lashings for trying to do without for so long.
-Casey
Kofi:
270 pF will set the pole in the FR back to 79KHz as in the original design.
A pole at 20 KHz means that gain and phase will change well within the audible range.
Gastón
270 pF will set the pole in the FR back to 79KHz as in the original design.
A pole at 20 KHz means that gain and phase will change well within the audible range.
Gastón
True, but I bet it goes for more cash at the final bell.
Excellent! My micas should arrive on Thursday, so I'll solder them in then.
Also, would it be possible for me to find a decent oscilloscope for audio work at around $100? I'll actually try and bid on the one you recommended, valveitude, but if that falls through, maybe you could recommend a good, cheap source?
For example, there's this one, but I don't know if it would be suitable.
So I guess I'm asking that if I were looking for a cheapie, what are the features and specs I'd need to accomplish the basics?
Kofi
Very suitable.
You want 50MHz minimum, 100MHz would be better, at least two channels. Most scopes have a built-in calibrator which makes a fine square wave source.
You want 50MHz minimum, 100MHz would be better, at least two channels. Most scopes have a built-in calibrator which makes a fine square wave source.
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