Modulus-686: 380W (4Ω); 220W (8Ω) Balanced Composite Power Amp with extremely low THD

[tomchr]

26 dB will be the default gain.

Tweakers can change the gain by changing a resistor. 20 dB is the minimum supported gain and is reached by removing the gain resistor.

I suppose some will want to know what the maximum is. For all practical purposes, there isn't much of a limit. You'll start to reduce the bandwidth of the amp somewhere around 50-60 dB. At some point the OPA1642 will run out of loop gain. The max gain is basically limited by the amount of noise you are willing to tolerate in your system.
I don't see any reason at all to go beyond 33-34 dB as that will allow a 900 mV source, such as a tablet or phone, to drive the amp to clipping. You better not have a finger slip when you're trying to turn up the volume in your music app.

I will always recommend to go with the lowest gain possible as that gives the best noise performance. Go with 20 dB if your source has enough oomph. 26 dB is a happy medium that gives good noise performance and allows common consumer line level to drive the amp to clipping.

Tom

[HarmonicTHD]

Originally Posted by tomchr

26 dB will be the default gain.

Tweakers can change the gain by changing a resistor. 20 dB is the minimum supported gain and is reached by removing the gain resistor.

I suppose some will want to know what the maximum is. For all practical purposes, there isn't much of a limit. You'll start to reduce the bandwidth of the amp somewhere around 50-60 dB. At some point the OPA1642 will run out of loop gain. The max gain is basically limited by the amount of noise you are willing to tolerate in your system.
I don't see any reason at all to go beyond 33-34 dB as that will allow a 900 mV source, such as a tablet or phone, to drive the amp to clipping. You better not have a finger slip when you're trying to turn up the volume in your music app.

I will always recommend to go with the lowest gain possible as that gives the best noise performance. Go with 20 dB if your source has enough oomph. 26 dB is a happy medium that gives good noise performance and allows common consumer line level to drive the amp to clipping.

Tom

My source provides 4VRMS, so I guess the 20dB option for me? Do I need to remove the resistor or can the 20dB option be ordered directly?

One further question. Do I need a soft start board for the 500VA or will the household circuit breaker be able to handle it?

Thx

[soongsc]

I think it depends on cap size? But possibly you may just see the lights dim briefly?

[HarmonicTHD]

Originally Posted by soongsc

I think it depends on cap size? But possibly you may just see the lights dim briefly?

I use the size which was recommended for Tom’s Pwr86 supply, which has 2x22.000uF as main caps. So would that throw the circuit breaker?

[jacek_k_wawa]

@tomchr

What about driving electrostats which impedance dips below 1 Ohm at 20kHz? Would that be a problem for either stability or performance of your amps?

rgds
-jacek

[AndrewT]

Originally Posted by HarmonicTHD

............. Do I need a soft start board for the 500VA or will the household circuit breaker be able to handle it?...........

My answer is very definitely.
A 500VA drawing maximum power from a 220Vac supply will run on a T2.5A fuse.
But you don't run your transformer at maximum output. You will find it operates on a T2A fuse without blowing even when having a drunken party.

Relying on a big fuse (or distribution board MCB/fuse) increases massively the time that a fault could draw enormous current until the fuse/breaker opens.

It's that time delay that increases the risk of fire, especially if left unattended while you go and answer the phone, or hang out the washing, or etc.

A close rated fuse is safer. And that needs a soft start.

[tomchr]

Originally Posted by HarmonicTHD

My source provides 4VRMS, so I guess the 20dB option for me? Do I need to remove the resistor or can the 20dB option be ordered directly?

If you let me know at the time you order (please via email AND the notes field on the order) I can certainly pluck those resistors off for you and deliver 20 dB boards for you. No additional charge.

Originally Posted by HarmonicTHD

One further question. Do I need a soft start board for the 500VA or will the household circuit breaker be able to handle it?

Your house circuit breaker will be fine. As Andrew points out, it's the rating of the fuse in the amp that you have to worry about. To make the fuse survive the inrush, you need to size it up a bit. This means it won't provide as good protection in case of "soft faults". I'm not overly concerned about that. If you want protection against soft faults (like arcing), you don't use a fuse anyway. You use an arc-fault interrupter or some other circuit. The fuse is to protect against the cases where wiring has frayed and shorted to ground or your supply caps have shorted out. For that, a fuse that's large enough to survive the inrush will work just fine.

In my view, the purpose of a soft start is to extend the life of the transformer. The loud hum you hear right when the transformer starts up is the primary stretching. Not by much, but enough that over 25-30 years it'll wear through the enamel on the wiring. Then the primary shorts out and the fuse blows.

A soft start board is definitely on my radar. For such a simple circuit, it's actually surprisingly hard to get it right. Many of the existing circuits out there drive low-voltage relay coils directly from the mains, which I really don't like. Others forget that the peak power dissipated in the inrush limiter (resistor or NTC) is enormous. Anyway. It's just one of these little five-component circuits that requires lots of thought and data sheet reading.

Another circuit on my radar is a speaker protection circuit. That one I'm further along with.

Originally Posted by HarmonicTHD

I use the size which was recommended for Tom’s Pwr86 supply, which has 2x22.000uF as main caps. So would that throw the circuit breaker?

It hasn't for me. The lighting circuits in my lab are on a 15 A breaker.

Originally Posted by jacek_k_wawa

What about driving electrostats which impedance dips below 1 Ohm at 20kHz? Would that be a problem for either stability or performance of your amps?

I've tested the stability with 8 Ω || 1 uF. No issues. Usually what saves electrostatic speakers is that the impedance rises above 20 kHz, so it's not a pure capacitor.

Originally Posted by AndrewT

It's that time delay that increases the risk of fire, especially if left unattended while you go and answer the phone, or hang out the washing, or etc.

I think that's overdramatizing things a bit.

I do agree that you should always have a fuse in the amp itself and that this fuse should be rated for the lowest current practically possible. This may mean that the fuse is rated for a higher current if you don't use a soft start than if you do.

In either case, the rating of the fuse will be lower than that of the circuit breaker in your household breaker panel. The fuse in the amp should also be a type where a wire melts and breaks rather than a thermal circuit breaker. There we certainly agree.
A typical glass fuse of the slow-blow/time-delay type will typically last a minute or so at 100 % overload (AFAIR). A typical thermal circuit breaker will take an hour to trip under those conditions. A glass fuse will blow within a few ms on a hard fault.

Always use a fuse. Get a fused IEC inlet. That makes things really easy. Use a slow-blow/time-delay fuse with as low a current rating as practically possible. This means the fuse should not blow when the amp is turned on and should not blow when the amp delivers the full rated power into the lowest specified load impedance.

Once I get a little further with the design documentation, I will make specific recommendations regarding the fuse rating with and without a soft start.

Tom

[Earl Grey]

Originally Posted by tomchr

Another circuit on my radar is a speaker protection circuit. That one I'm further along with.

Don't suppose you'd hazard a guess as to the likely size of the board (or tentative timeline ) for this, would you, Tom?

I'm finally building up my 2 x Mod-86 / Power-86 and have all parts but the enclosure. I will definitely order the protection board and would hate to find that my enclosure or layout doesn't easily accommodate retrofitting it.

[HarmonicTHD]

Originally Posted by tomchr

If you let me know at the time you order (please via email AND the notes field on the order) I can certainly pluck those resistors off for you and deliver 20 dB boards for you. No additional charge.



Your house circuit breaker will be fine. As Andrew points out, it's the rating of the fuse in the amp that you have to worry about. To make the fuse survive the inrush, you need to size it up a bit. This means it won't provide as good protection in case of "soft faults". I'm not overly concerned about that. If you want protection against soft faults (like arcing), you don't use a fuse anyway. You use an arc-fault interrupter or some other circuit. The fuse is to protect against the cases where wiring has frayed and shorted to ground or your supply caps have shorted out. For that, a fuse that's large enough to survive the inrush will work just fine.

In my view, the purpose of a soft start is to extend the life of the transformer. The loud hum you hear right when the transformer starts up is the primary stretching. Not by much, but enough that over 25-30 years it'll wear through the enamel on the wiring. Then the primary shorts out and the fuse blows.

A soft start board is definitely on my radar. For such a simple circuit, it's actually surprisingly hard to get it right. Many of the existing circuits out there drive low-voltage relay coils directly from the mains, which I really don't like. Others forget that the peak power dissipated in the inrush limiter (resistor or NTC) is enormous. Anyway. It's just one of these little five-component circuits that requires lots of thought and data sheet reading.

Another circuit on my radar is a speaker protection circuit. That one I'm further along with.



It hasn't for me. The lighting circuits in my lab are on a 15 A breaker.



I've tested the stability with 8 Ω || 1 uF. No issues. Usually what saves electrostatic speakers is that the impedance rises above 20 kHz, so it's not a pure capacitor.



I think that's overdramatizing things a bit.

I do agree that you should always have a fuse in the amp itself and that this fuse should be rated for the lowest current practically possible. This may mean that the fuse is rated for a higher current if you don't use a soft start than if you do.

In either case, the rating of the fuse will be lower than that of the circuit breaker in your household breaker panel. The fuse in the amp should also be a type where a wire melts and breaks rather than a thermal circuit breaker. There we certainly agree.
A typical glass fuse of the slow-blow/time-delay type will typically last a minute or so at 100 % overload (AFAIR). A typical thermal circuit breaker will take an hour to trip under those conditions. A glass fuse will blow within a few ms on a hard fault.

Always use a fuse. Get a fused IEC inlet. That makes things really easy. Use a slow-blow/time-delay fuse with as low a current rating as practically possible. This means the fuse should not blow when the amp is turned on and should not blow when the amp delivers the full rated power into the lowest specified load impedance.

Once I get a little further with the design documentation, I will make specific recommendations regarding the fuse rating with and without a soft start.

Tom

Thanks for all the info.

As I want upgrade my two 286 to 686 I can now start ordering the 500VA, throw out the 300VA, possibly replace the 3A SB fuse in the IEC inlet and then order the 20dB 686 boards. BTW thanks for the option to take out the gain resistor. Actually I would get less components for the same price ;-)

[chrisb]

Power supply agnostic circuit architecture - gotta love that description