Old 300bpp Ringing

[Richard]

Just a quickie for Neal, Greg and others with the old 300bpp.

The following sw are top 2 with feedback off and bottom 2 with fb on, 1W/8R/1KHz,



Sine wave is similarly worse with fb on - very distorted at 20KHz at outputs over 10W with fb on but quite good up to 18W output with fb off.

With fb off it performs well - exactly 28W/8R/1KHz etc.

I'd have expected fb to possibly worsen the ringing on sw but generally improve the bandwidth.

Now, this amp has battery bias on the first stage. What I'd like to know - before I rip it out - is what is the sw like without battery bias? Is the way fb is now being applied causing this ringing or is the sw this bad using a cathode resistor?

Thanks

Rich

[NealG]

Hi Rich,

Don't know if you can recall we had a three way email conversation with Dr John about 3-4 years (!) ago about this, in the end I emailed Noel and he said it was normal if you tried to pass a 1KHz SW through the interstages. If you lower it to 200Hz you should find it is OK, this is due to the bandwidth limit of the Interstages I was told.

The battery BIAS doesn't affect it at all, I'll see if I can dig up the email thread and forward it on to you...

This is a classic case of measurement not reflecting how the thing sounds!

All the best.

Neal

[Richard]

Thanks for those old emails Neal. Gosh was it 2002.

Yes I remember our conversations at the time but didn't recall pics of how bad the sw was with fb. The pics without fb show a little overshoot and a cycle or two of ringing and you'd expect to lower the overshoot with the fb compensation components. Instead it looks totally out of control.

This result also clearly shows using sine wave as much reduced power bandwidth and increased distortion at higher f using fb. If this is a normal result for this amp then the only reason I can see for having fb is to lower op impedance and sensitivity at the expense of bandwidth.

In use it sounds better without fb but that's hardly surprising looking at both sine and sw waveforms

Is this hi-fi? Nope. I'll put it back to standard and see if it improves, or can be improved, with my limited tinkering.

Rich

[Ianm2]

better interstages perhaps may be fruitful.

[Richard]

OK, took off the battery bias and fitted a 1K cathode resistor bypassed with a 470uF Starget electro.

Without fb the sw and sine is the same as before.

With fb it is a lot better than before on sw as below 1W/8R/1KHz,



Also much better on sine with better bandwidth than without fb and 24W available at 20KHz before visible distortion of the trace.

Rich

[NealG]

Good one Rich, thanks for investigating, just as well the majority only ever use it without feedback

What does it look like with a 200Hz SW, the interstages are not going to pass one at 1Khz?

[Richard]

Hi Neal

With a 200Hz sw the overshoot and ringing is at the same amplitude but damps out more quickly with fewer cycles. Visually it's like going from the first of these traces to the second then another step again (or from the wedge shape to the countersunk screw to a nail head )

I can see the logic for sw testing at a lower f, the harmonics will be in the audio band, and probably all circuits ring at some point so maybe we should only be looking where it matters.

The curious thing with the battery bias is that it did make a difference to the trace. That suggests it's the battery's ac connection properties. Those properties may apply to the ac connection when used without fb also. Greg remarked on the difference in sound with different batts which seems to bear this out. I only did a cursory check on the bandwidth, with/without batteries without fb. The 1KHz sw was very similar but I'll get a listen later. With fb on the switch now makes a definite improvement to bandwidth whereas before it actually made it worse.

Rich

[Richard]

Hi Neal,

Firstly, a public thank you for this amp! I realised looking at this thread that it might seem I'm pulling it to pieces. Not so, we both know I'm very pleased, it's mint condition cosmetically, well built, sounds great and was a gift of a price. I also appreciate the work of listening and fitting good parts that you've put into it.

A bit of history, 5 of us had a listen to several amps here in my system 4 years ago. When this one went on we all said "wow, that's nice" and ever since I've kept an eye out for one.

So, a bit strange to not hear it as expected on first listen using the same CDp, speakers, cables, room etc.

Reverting to cathode resistor first stage has fixed it though. Superb without feedback on. Huge soundstage beautifully laid out in 3 dimensions and lush with detail. Listened for several hours yesterday. Started with rock to see if it was happy driving the speakers and room. Progressed to Janis Ian (water colours has never sounded like that before and I'll try it again sober ), Joni Mitchel, Cat Stevens, Tracy Chapman etc.

Still poor with fb switched in, very different, it goes flat and grey, so not much point having it really.

Matched the JJs and Chelmers and they're fine. Got a new set of EH from Mark at Hotrox so had fun rolling 3 types of 300Bs as mentioned in Paul's thread.

Only other thing I did was to take off a divider on the input Neal, 91k series with 1M // existing 100k = 0.5x. Did you fit that or was it on before?

Thanks again,

Rich

[Greg.]

Well done Richard. Getting us all going again . Time to get the iron out and return to trial comparing battery bias with standard. It'll be interesting to see what others find.

Best wishes,

Greg

[NealG]

You're welcome Richard, glad you like it and that its gone to a good home and someone who appreciates it.

TBH I fitted the battery BIAS thought is sounded better and then forgot about it, as I don't use it with feedback it never occurred to me to actually check it!

You've made me curious now, I'll have to revert back and listen!

I forgot about the PD, I used it to lower sensitivity in my system, far too much gain with the f/b switched out.

I'm surprised about the Chelmers, they are the originals I bought for my amp back in '94 (?) They are tired and when we listened to them at Greg's they sounded it! (or so we thought!)

I'm off to warm up the iron!

All the best.