First One - mosFET amplifier module

[Mark Verhoeven]

P.S. where (and how) to measure output bias current?

 

[Lazy Cat]

P.S. where (and how) to measure output bias current?

The simplest way with an A-meter in series to positive supply wire, it should show collective ie. total 280 mA (220 mA output stage, 60 mA the rest of the circuit) DC current. Use 10 A DC current range and correspondent 10 A plug on multimeter. 😉

 

[Mark Verhoeven]

Thanks!

I am just checking and adjusting for DC offset and VAS bias current, now I have another question:

For adjusting VAS bias current, is it O.K. to only measure between TP1 and TP2, or should I also check between TP3 and TP4?

 

[Lazy Cat]

TP1-TP2 have the same function as TP3-TP4 so it doesn't matter which one you choose, one measurement is enough. TP1 is on +VDC potential, TP4 on -VDC, so the measurement to be even easier you can connect mV meter's positive probe to +VDC by clamp and handle only the negative probe to TP2 and set to 120 mV which correspond to 12 mA of VAS bias current.

 

[Mark Verhoeven]

I did solder 2 thin wires to TP1 and TP2, that was in my opinion the safest option.

Did one amp (mono) until now. Directly after powering up it shows around 11 mA and it creeps to approx. 15 mA after quite some time of idle. (approx. 15 minutes). 12 mA is reached after approx. 30 sec - 1 minute. DC bias is 0 all the time. So I assume it's biased nicely.

How important it is to check and correct output bias current? (didn't touch the setting since buying the amp from you)

 

[Lazy Cat]

In production they're using the beast from the pic so pre-calibration should be done OK, but personally make measurements check prior installation in the chassis, just to be 100% sure. You can leave the output bias (TR3) untouched. 😎

 

[Mark Verhoeven]

Here I am again...

It's a bit hard in my opinion to set VAS Bias as after switching on the amp, it creeps up quite quickly, specially the first 5 - 10 seconds. When should it read 120 mV, after 1 second, after 5 seconds, after 10 seconds? As I have set it now, it reads approx. 120 mV after 2 - 3 seconds after switch on and it creeps to 130 mV quite quickly, after that it creeps less quickly.

Please let me know if this is fine. If yes, I will setup the other amp the same.

Thanks,

Mark.

 

[Lazy Cat]

Hi Mark

From powering on VAS bias goes slowly up from 12 mA (ie. 120 mV between TP1-TP2) to 15 mA (ie. 150 mV between TP1-TP2) and stays there. That is completely normal, meaning only 3 mA difference from cold start to normal operating conditions.

VAS transistors are very happy to have working point at Ic=15 mA since hFE is optimal and linear at that point.

If you need auto bias circuit, which is completely meaningless here, than we would need OP amp control circuit with some low pass filtering etc.

First One is simple but highly effective amplifier, especially in SQ domain. Look to the first post for technical data.

As you can see from the picture above, production uses coolant regulated heatsink, having 23°C, so they simulate powering up conditions all the time. This enables them to save time and calibrate more modules per day, since they don't have to wait for the modules to thermally stabilize.

As you don't have this option I suggest to set VAS bias to 15 mA when heatsink is thermally stabilized in app. 20 min.

L.C.

 

[Lazy Cat]

So far we are nearly approaching 200th module and for that number two assembeld DC Sense modules are reserved and will be included as a gift.

Best regards L.C.

 

[Mark Verhoeven]

Thanks LC! That's probably the best thing to do for me, having the amp powered up for some time and set for 15 mA. 🙂

 

[irribeo]

I thought LC effort regarding calibration F1 was to ensure calibration already took place in a more precise manner then performed with 20 euro multimeters?

 

[Lazy Cat]

Not true. The aim of calibration in production is to set the module within the limits of technical specs. They use analog meters as seen from the pic, so 20 EUR multimeters could be more precise, but analog meters have different role here.

If DC offset is -5 mV or + 5 mV that is completely irrelevant for proper amp operation. If someone would like to set it to 0,1 mV it is still possible, I did it myself a lot of times, but than you have to use FLUKE.

Again, for proper amplifier operation it is complete nonsense to have DC servo or autobias circuit if amplifier itself operates within +/- 5 mV DC offset and 15 mA (+/-2 mA) VAS bias. DC servo and auto bias usually have its sound signature and that is worse situation than their main purpose.

 

[BYRTT]

Not true. The aim of calibration in production is to set the module within the limits of technical specs. They use analog meters as seen from the pic, so 20 EUR multimeters could be more precise, but analog meters have different role here.

If DC offset is -5 mV or + 5 mV that is completely irrelevant for proper amp operation. If someone would like to set it to 0,1 mV it is still possible, I did it myself a lot of times, but than you have to use FLUKE.

Again, for proper amplifier operation it is complete nonsense to have DC servo or autobias circuit if amplifier itself operates within +/- 5 mV DC offset and 15 mA (+/-2 mA) VAS bias. DC servo and auto bias usually have its sound signature and that is worse situation than their main purpose.

Also because the PCB only need input/output/power/heatsink it must be good to know module is tested before shipping sometimes very long distance.

To calibrate to 0,1mV precision as you did lot lot of times, well isn't it the satisfying care a diy can give such a amp ...😀

 

[Lazy Cat]

Also because the PCB only need input/output/power/heatsink it must be good to know module is tested before shipping sometimes very long distance.

To calibrate to 0,1mV precision as you did lot lot of times, well isn't it the satisfying care a diy can give such a amp ...😀

Modules are calibrated and power tested at 20 kHz 100Wrms/4 Ohm. 😉

0,1 mV DC offset is in preamp's domain and it was just tour de force test no deeper meaning here. 😀

 

[Lazy Cat]

Hi Cat. How are my Black Mamba's .

Your Black Mamba is alive and looking dangerous. Over the weekend will be playing music and be send to you on Monday. 😉

All chassis sold. 😀

One assembled First One sent to UK, one to Sweden, one to Singapore, one will be send to you and few locals already in production. 😎

Local Multimedia Show is taking place in Ljubljana from 3.-5.10., where First One V1.3 will be unveiled and presented, stay tuned for comments. 🙂

L.C.

 

[mikvous]

Just received my modules and black chassis.First comment regarding packaging
and quality of the chassis is that they are first class.
I hope that in a few days I'll have it assembled and compared with my other SSA
amps.
Thanks a lot LC
Mikvous

 

[Mikha65]

And yes, the FO arrived to Sweden in perfekt condition.
It looks like a dream and sounds like heaven.
It's very hard not to get emotional about this amp.

My system is not in action so I took FO to my Audiobuddy.
Short story is that after listening to a couple of songs his McIntosh
is up for sale. Mc is 5 times the price of FO .

His words: " I can't believe how FO slaughtered the Mc ".
We listened for 6 hours last night.

His system is Audio Aero, Ear, Clear Audio, Magnepan,Sablon Audio.

Ladies and gentlemen : Slovenia 12 points

Big thanks to Andrej and Andrej

Cheers,
Mikko

 

[Lazy Cat]

Khhm, khhm well to be clear this was not payed advertisement. 🙂

Thanks for the opinion Mikko. Enjoy. 😉

 

[Drsah]

Can we see Black Mamba?

 

[Lazy Cat]

First One Black Mamba

Finally Black Mamba swings around, looking dangerously on the hood