Official LYNX Power Amp builder’s thread

[Jan Dupont]

Opamp in general;

I can say that the OPA627 doesn't fit to my music style (old and new guitar rock). The OPA134 is a good and cheap choice.

However I must say, that I do use the old LF356 Opamp for my woofers

 

[Ragnwald]

Hi

While waiting for the PCBs, I´m designing the case.

The output transistors are all sitting on the PCB and that´s somthing new for me. I come to wonder how it´s intended to be mounted, for best heatsink.

Standing on the shortedge with the transistorhalfside upp and transistors pointing left and right would make best.

It means a relatively short but high heatsink ?

 

[Jan Dupont]

Ragnwald;

If you use a proper heatsink and heat conduction paste, it doesn't matter if they are turned up/down or left/right

 

[Jan Dupont]

I have updated the Quick Assembly Guide

Added more info on components, updated text in SOA protection, drawings with test points added, updated text in First test and added drawing for basic wiring.

 

[olev]

Could you give some general information on how much power (current) the board takes?

I mean if I want to get the specified 150W from 8 ohm - that calculates to 31.6 Vrms, 44.7V peak, 3.95 amps. So pure audio side needs 44.7x3.95=177 VA. How much do all the transistors take etc ... ofcourse one could go out and buy the largest transformer available but what are the specifications (with a little headroom) for an optimal transformer? From documentation 300 to 500W transformer is a little wide specification.

Thanks,
Olev

 

[Ragnwald]

Thanks Jan

Hm... tell us about old LF356 Opamp

 

[dexter]

Jan,

Excellent assembly guide, it's better than any professional kit that I'd read.

Please tell me how the lf356 sounds?
I have some old ceramic base lf 356 and never use it

also can I make some suggestipn ?

Maybe you can add PSU bleeder resistor on your psu

rgds,

 

[Jan Dupont]

olev;
Your calculation is right and a 200W toroid can do the job for one channel. However if you intend to play loud (which this amp is build to) you may get into problems keeping the Capacitor Bank fully charged. That's why many (my self included) recommend to use a toroid that is at least 1.5 times bigger than nominel output Wattage.
I will put something in the assembly guide

Ragnwald and dexter;
Just one of the most used opamps in the old days
I think it's exelent for rock music, however I will not recommend it for more delicate classic music.
It's still easy to get and it's cheap. Use a 8-pin DIL socket on the pcb, and make your own listening test

 

[Ragnwald]

ACD; Do that mean you nead toroids of 600VA pr. channel to get reliable 400W output pr. channel into 2 Ohms ?
Going for supply voltage of +/- 55 Volt DC, as in your exampel.

regards

 

[Jan Dupont]

Ragnwald;
If you are going to use such low impedance speakers, you don't need +/-55 VDC. You can achieve 400W/2 Ohm with +/-43 VDC Rail Voltages. But yes I would recommend a 600W toroid with 2.000uF per Rail per Ampere.

 

[olev]

Ok you answered 400W into 2 Ohm. What do you recommend for 275W into 4 Ohm.

 

[Jan Dupont]

olev;
Rail Voltage +/-50 VDC
Toroid 400W with 2.000uF per Rail per Ampere.

 

[crt]

halo, jan

All the pair transistor should be matching?
or just some pair transistor only?

And what gonna happened if I don't matching the transitor? is gonna sound BAD?

sorry for my stupid questions

 

[Jan Dupont]

crt;

Using matched transistors is always an advantage, however it is not necessarry.....
I didn't match the trannies in my latest LYNX builds, and frankly I can't hear any difference!
By the way... Try to match the MPSA's... You may need 100 of each to get one match.

And then you all ask why I used these trannies in the LYNX?
Because it was developed mostly by ear and less by measuring.
I don't care about 20-30 mA DC offset on the output, if the sound is good

 

[Ragnwald]

Hi Jan

Ragnwald;
If you are going to use such low impedance speakers, you don't need +/-55 VDC. You can achieve 400W/2 Ohm with +/-43 VDC Rail Voltages. But yes I would recommend a 600W toroid with 2.000uF per Rail per Ampere.

My speakers are all spec 8 Ohms, but they are certainly not all the way 20Hz to 20KHz.

So my intention is to design the powersuply/amplifier for 8 Ohm speaker drive.

In your introductions and specifications, you stated that the amp driven with +/- 55VDC should be abel to produce 150W into 8 Ohm and 275W into 4 Ohm and 400W into 2 Ohm.

If the amplifier clips att 150W resistive, can I expext it to clip at no less than 1,8x into 4 Ohm resistiv and also be a 2 Ohm capable amplifier ?

I´m keeping asking, for this is for me the most expensive part of the building process.


regards

 

[crt]

Thank's Jan

I will use lf356, because i'm on ROCK line.

 

[crt]

for 33pF, can i subtituse this caps with lower or higher capacitance?
47pF or 22pF

and for 22uF can i use bigger capacitance too? 68uF polar. is this part should be non polar?

thank's

 

[Jan Dupont]

Ragnwald;
Don't worry
If you make a good big PSU with lots of uF, there will be no problem....
Let's say that we set the Current limit for the protection circuit to 3.5A per output device (as described for 55 VDC Rails in the Assembly Guide) . This gives a total of 14A output per rail.
14A into 2 Ohm=28Vout. > 14A*28Vout=392W + the dynamic headroom build into the protection circuit.

 

[Jan Dupont]

Thanks for payment, cocolino

 

[Jan Dupont]

crt;
Rock'n Roll

I found that 33pF was ok when I first designed the amp. But you can try the nearest lower or higher value. Can't remember the result with other values, as it was many years ago....

You shouldn't go much higher than 22uF in the protection circuit unless you lower the max acceptable Current through the output devices. These two caps are the delay before the protection cuts in (allowing dynamic peaks to pass through), and they also stabilize the protection circuit, so it doesn't goes on/off rapidly when the Amp is pressed to the limit........