symmetric amplifier

[Rambi]

Hi all,

Thanks for all the helpfull comments and suggestions so far!
I thought I'd let you know what I'll do with them:
My plan is to first *finish* an amplifier (casing and all I mean), so I'll integrate the suggestions that are easiest to incorporate first:
- Lower value for R56 (100 instead of 1200)
- base resistors for Q9/Q109
- BC550C/BC560C in LTPs (to be honest: schematic is inaccurate since I already used those. The reason I put a cascode in the LTP in the first place was the slightly over 40V power supply, a little bit too close for comfort to the limits of the BC550 & BC560...)

I don't need alterations around ccs LED and cascode zener for reasons of reducing hum, that's already non-existent anyway ('pre-stage' and 'power-stage' are already somewhat decoupled by a LPF of R40/C42, R42/C43), so I'll leave those suggestions for v2.0
I already tried the lower R56 value (very easy modification of course) and was pleasantly surprised by the effect
Much more detailed! Switch from OpAmp based to solid state yielded much more details in mid & high. This simple change resulted in even more details but predominantly in 'lower' regions, e.g. cello and the lower keys of the piano ('better defined attacks of strings' is the best way I can think of to describe it). Some recordings are also less 'harsh' than before.

For the exisiting prototype the addition of base resistors for Q9/Q109 can be retrofitted relatively easy by cutting two tracks and drilling four holes. Since a decent amplifier needs two channels (either in the same case or separate, but I don't want to start that discussion here ) it's very easy to include it in a new PCB as well (minor mod).
All other suggestions require (and certainly deserve!) more investigation & design.

So why finish an amplifier first? Well, I don't expect to ever finish with tweaks and improvements and it would be a shame if that would prevent me from ever finishing a complete amplifier.
Also my ultimate goal is a bi-amped system, actively if possible, but that's in a far and distant future. Since that implies a second amplifier I already have a valid excuse to look into & try the other suggestions .
Which brings me to the final (last but certainly not least) reason: The SWMBO factor (or HWMBO in my case). At least finishing a complete amplifier is a very persuasive manner of justifying more time & money spent on this subject ("What? are you telling me that you don't want to enjoy better music?" )

Next steps:
- build the other channel.
- put it in a nice 19" case (hmm, that makes it sound so easy)
- try not to forget to take pictures along the way
- enjoy/work on v2.0/think of preamp (all at the same time)

Wow, what a hobby!

Remco

P.S.: Sorry for the long post, but I feel that the thoughtful and helpful suggestions at least deserve some explanation as to what I'll do with them.

[Rambi]

Originally posted by dkemppai

Cool, here are a few images. (I hope you can find that camera to snap a few to post also )

The measurements are pretty self explanatory, as to frequency, phase, and gain (can be calculated from peak to peak) Blue trace is output signal into 8 ohm 100W non inductive resistor. Yellow trace is input into amplifier.

I was going to grab a few more, but I left it run at high power a little too long... ...POP! SNAP! followed by orange flame from the fets, and lots of magic smoke. So, until I rebuild (In about an hour) you'll have to settle for the 100Khz and single 10Khz traces...

Also, the amp was running at +- 50 Volt rails, +- 70 volts seems to improve linearity at high frequencies.

I'll stick to about 40V... Never let the magic smoke escape!, you should know better than that
Nice pictures!
Cool to see the phase shift increase between 10 & 100 kHz .
(and the 'wobbles' on the peaks of the triangle @100kHz)
If I manage to get my hands on a digital camera you'll see 'real' scope pictures (including the 'Tektronix 434 storage oscilloscope' I mean ).
Were you already able to listen to it and find out how it sounds?
(Just curious, since you apparently use fets)

[timc]

Hi Rambi,

Is there a slight error in the DC Servo?
Shouldn’t the centre point of the zenders be connected to ground and +ve input of opamp instead of the –ve input.

Regards,
Tim

[dkemppai]

Originally posted by Rambi


I'll stick to about 40V... Never let the magic smoke escape!, you should know better than that

Were you already able to listen to it and find out how it sounds?
(Just curious, since you apparently use fets)

This is the second set of Fets I've burned up, so there's lots of magic smoke on the walls to help me think
(Still can't figure out why my wife bought me that fire extinguisher, fets ALWAYS self extinguish Trust me, I know)

Yes, I use fets for the output devices, but the rest of the amp is BJT. No, I have not had a chance to listen to it. Before I hook speakers to it the bias voltage for the output fets needs some temp compensation. It also needs the integrator (Although, the trim resistor I'm using for DC offset seems to work well). I've only spent a few days doing the design/test, so there's lots that needs to be done. I've got my first amp sitting here also, and I plan to do some 'side by side' comparisons.

You say you've got a board designed? How much for parts for your amp? Are they avaliable from common supplers in the US? It would be nice to hear how yours sounds. It would also be nice to see some pics of your board and stuff, as you get them!

Also, Looking at your schematic (Nice layout), You may try a resistor from base to emitter on both Q9 and Q109. Something in the range of 470 to 100 Ohms. Darlington pairs often use them to help with the slew rate...


-Dan

[grimberg]

dkemppai,

What is the brand and model of the oscilloscope you are using? I'm currently shopping for one.

[Rambi]

[

Originally posted by timc
Hi Rambi,

Is there a slight error in the DC Servo?
Shouldn’t the centre point of the zenders be connected to ground and +ve input of opamp instead of the –ve input.

Regards,
Tim

I don't think so, the diodes (not zeners) are just there to prevent the input from rising above or below the supply voltages of the opamp (not very likely, but you never know).
Since the integrator is connected to the input side of the LTPs it needs to be a 'negative' integrator as well (i.e. positive input voltage causes negative going ramp on output of integrator).
Besides, connecting it the wrong way around would cause a DC offset, not cancel it, that would be noticeable soon enough

Remco

[Rambi]

Originally posted by dkemppai

You say you've got a board designed? How much for parts for your amp? Are they avaliable from common supplers in the US? It would be nice to hear how yours sounds. It would also be nice to see some pics of your board and stuff, as you get them!

Most stuff is 'ordinary' and shouldn't be hard to get in the US. Don't know about the BC type transistors, should be no problem to replace them with other types of small signal transistors and maybe MJE340/350 for the BD139/140 types?
Sanken transistors are the most expensive parts (for populating the board, i.e. not counting heatsinks, casing, toroids, etc.)

BD911/BD912 (which I use for prototypes) are a few euros per piece if I buy them in a store, but on-line store like Reichelt seems to have them for about 50 eurocents (roughly the same in dollars).

Remco

[dkemppai]

Originally posted by grimberg
dkemppai,

What is the brand and model of the oscilloscope you are using? I'm currently shopping for one.

http://www.tek.com/cgi-bin/wtx.cgi?w...rameSet=Escope

A bit spendy, but nice! I've also used the TDS1000, TDS2000 and TDS200, and TDS400 series. All very nice. (TDS1000 Starts at about $1000)

-Dan

[Jeff R]

I have a question about the servo op amp, U1. There is only a capacitor in the feedback loop. If the op amp has a dc output offset voltage (and they all do) it is possible for a dc voltage to develop across the integration capacitor and ultimately build up so large as to saturate the op amp. Typically, a large value resistor is paralled across the capacitor to provide a path for the dc to discharge. You have a pretty large Rin right now, and the LF411 is not exactly a low offset voltage op amp, so I am concerned about this charge buildup.

If this is a problem, you may need to lower R80 to maybe around a 10 - 100K and parallel C80 with 1 to 10 M. The output network may have to be rescaled as well.

[peranders]

Originally posted by dkemppai
I was going to grab a few more, but I left it run at high power a little too long... ...POP! SNAP! followed by orange flame from the fets, and lots of magic smoke. So, until I rebuild (In about an hour) you'll have to settle for the 100Khz and single 10Khz traces...

Also, the amp was running at +- 50 Volt rails, +- 70 volts seems to improve linearity at high frequencies.

Very dangerous to drive the amp at very high frequency with no or little load, even dangerous with load. I have made the same mistake myself. +-47 V, BD317, BD318. No load, 200 kHz = smoke! The output transistors started to conduct at the same time. Talk about fireworks in the fuses!