Solid state power amp; impedance question

[Bernardduur]

Hey all!

I got this great Ampeg amp (B410) that sounds killer...... my SVT next to it is louder and more heavy, but the overall tone is quite the same. It helps that the B410 has a sweepable mid range section I guess......

Now the issue is that it can only be used with a 8 ohm load. I do own a lot of 8 ohm cabs but I want the freedom to also use it on a 4 ohm load (as my main cab is 4). Also, my thoughts were sparked on how this prime impedance is set....... I just can't find an explanation I can get

So, it is mainly a design issue or is it a BIAS or heat issue (do I need a bigger heatsink on it )

Here is the schematic

B115 B410.pdf

I do understand that such a poweramp works like an comparator and that the gain is derived from the feedbackloop resistor (18k) and the resistor to ground (150R + 330u). Would halving the gain (for example, changing the feedback resistor to 9k) make the current with a 4 ohm load be comparable to that of the original amp with an 8 ohm load?

[Dave H]

If it's designed for 8 ohms min it will have a higher voltage power supply (than an amp designed for 4 ohms) and will probably overheat and blow its power transistors if you use it with 4 ohms. An amplifier designed for 4 ohms will have a lower voltage, higher current power supply.

[Jazz P Bass]

The amp is capable of delivering what, 100 watts at 8 ohms?

So at 4 ohms you are expecting the amp to be able to deliver 200 some watts?
(when it is clearly stated "8 ohm minimum" )

It's all in the design.
If the power supply can supply it, if the output section predrivers can deliver it, if the output transistors can safely pass it.......

[Bernardduur]

The amp is capable of delivering what, 100 watts at 8 ohms?

So at 4 ohms you are expecting the amp to be able to deliver 200 some watts?
(when it is clearly stated "8 ohm minimum" )

It's all in the design.
If the power supply can supply it, if the output section predrivers can deliver it, if the output transistors can safely pass it.......

No, I don't want any extra power, I want the amp to function on 4 ohms (and deliver the same 100W as it does now to 8 ohm)

I want to know if such an amp can be altered...... my knowledge of these amps is low and is increasing as of today (found some nice reading materials online)

[Bernardduur]

If it's designed for 8 ohms min it will have a higher voltage power supply (than an amp designed for 4 ohms) and will probably overheat and blow its power transistors if you use it with 4 ohms. An amplifier designed for 4 ohms will have a lower voltage, higher current power supply.

Yes, that is what I found out today; by lowering the RMS (peakvoltage * 0.7) I can lower the wattage and the total power of the unit.....

I thought on replacing the bridge rectifier with two diodes to cut the voltage (half wave vs full wave rect).

[R.G.]

I thought on replacing the bridge rectifier with two diodes to cut the voltage (half wave vs full wave rect).

Unfortunately, it doesn't work that way. Half wave gives the same peak voltage as full wave, but make the filter capacitors half as effective, so doubles the ripple.

You have two issues that need fixed. One is that the output transistors will have to conduct twice the current at 4 ohm loads and live, and the second is that there is protection circuitry in the power amp to keep them from dying at some unspecified current/voltage conditions.

The first issue is easy to fix in concept, and it involves the power supply, as you've guessed. The simplest thing to do is to replace the power transformer with one which has a lower secondary voltage. This is electrically simple, but will require finding and then mechanically adapting a different and smaller transformer.

The second issue is harder. The schematic shows some circuits which monitor the output voltage and the output currents and then shunt the drive to the output transistors when the current and/or voltage get dangerous for the output transistors. Even with a lower supply voltage, this particular circuit might step in to save the output devices when it's not needed. It's possible to recalibrate/modify this circuit, but simpler to put on more power transistors in parallel with the existing ones so the current sense sees less current per transistor.

[olddawg]

Just run another 4 ohm cab in series with your existing cab or add a 4 ohm resister of the appropriate wattage in series. Decades ago I had a problem with Yamaha amps blowing up with Infinity speakers. I was an authorized servicer for both. The salesmen were selling them as a hi end combo. Yamaha was blaming a Infinity for dropping below 4 ohms at certain frequencies and Infinity was saying that they were not responsible for Yamaha being that picky. The retailer didn't want to take them pack. I ended up mounting a 2 ohm resistor in each Infinity can on the inside of the positive binding post. No more problems, customer couldn't hear the difference, and everybody was happy. ( As long as I didn't tell them what I did).

[Jazz P Bass]

No, I don't want any extra power, I want the amp to function on 4 ohms (and deliver the same 100W as it does now to 8 ohm)

I want to know if such an amp can be altered...... my knowledge of these amps is low and is increasing as of today (found some nice reading materials online)

O/k.

As long as you don't turn it up all of the way, you are good.

It would be best to test the rig & find out at what volume level setting 100 watts into 4 ohms occurs.

[loudthud]

It will be far far easier, cheaper and faster to put together an 8 Ohm load than to modify the amp.

[g1]

Or you can point a fan at it, don't push it, and take your chances .
It really comes down to a heat issue.

[Dave H]

I wouldn't run it at 100W into 4 ohms. It would be too much heat for the power transistors to dissipate. They will have 1.414 x the current and more voltage across them than at 100W into 8 ohms.

Use one of your 8 ohm cabs or put an 8 ohm speaker in your favourite cab.

[Dave H]

Or you can point a fan at it, don't push it, and take your chances .
It really comes down to a heat issue.

And have a box of spare power transistor handy.

Even with a fan cooling it the extra voltage and current could excede the safe operating area of the transistors.

[Bernardduur]

Ah, that would be my guess also; I learned that these amps are designed for this purpose........ and that transforming em around to 4 ohm is too much work . I'll stick with the 8 ohm and modify my rig.

Just love to learn from my own amps; never thought about SS amps to be this..... interesting. I've had a great read and must say that the smog and mystery behind these amps are slowly dissapearing........

Then one last question; if I would use it on 4 ohm, can it be altered in a way that it is more 'safe'?
- Bigger heatsink?
- Fan on the heatsink?
- Adding more output transistors (+ bigger heatsink) parallel to the ones already in the amp to split the current?

Or is this amp also designed to interact and make that amount of current unable?

@ RG: What are the parts in the schematic that check if the current / voltages are not dangerous to the amp?

[uneumann]

Ah, that would be my guess also; I learned that these amps are designed for this purpose........ and that transforming em around to 4 ohm is too much work . I'll stick with the 8 ohm and modify my rig.

Just love to learn from my own amps; never thought about SS amps to be this..... interesting. I've had a great read and must say that the smog and mystery behind these amps are slowly dissapearing........

Then one last question; if I would use it on 4 ohm, can it be altered in a way that it is more 'safe'?
- Bigger heatsink?
- Fan on the heatsink?
- Adding more output transistors (+ bigger heatsink) parallel to the ones already in the amp to split the current?

Or is this amp also designed to interact and make that amount of current unable?

@ RG: What are the parts in the schematic that check if the current / voltages are not dangerous to the amp?

You're simply risking failure if you run at 4 ohms. The whole circuit is spec'd and designed for that limit. It would be very time consuming to analyze all the ways the amp could fail at 4 ohms, and I don't think anyone will take the time to do it for you. Even if they did, could you be absolutely sure they caught everything? Even factory tests within the specs can turn up failure modes over time that no one expected.

To be safe, use a 4-ohm resistor in series with your 4 ohm load. You'll lose some power, but at least you won't have to check for smoke every time you hit a loud note.

It's not likely that a trivial fix (fan or heatsink) solves the problem. Think about your car with a red line at 8k rpm. Would you trust or expect someone to help you mod it to run at 16k rpm?

[J M Fahey]

Short answer: although in theory mod is possible, it implies replacing/rewinding the power transformer by one putting out 0.7X the present voltage and 1.4X the present current; the power amp by one with transistors with same factors (same power, 0.7X the voltage, 1.4X the current, not forgetting a properly set short protection) and half the power supply (diodes would be probably enough, you'd have to double capacitance, overrated voltage doesn't hurt).

You sure yu want to go that way?

In a nutshell: an amp designer works backwards: starts with desired power and impedance, calculates voltage and current requirements, then which and how many transistors can safely supply that, then which power supply can feed them.

As you see, starting with : "hmmmmm, boss wants a 100W into 8 ohms amp" or "boss wants a 100W into 4 ohms amp" leads to different solutions.

FWIW I don't remember having seen that exact model in person, but distinctly remember its bigger brother from the late 70's or early 80's , the SVT240 (or was it SBT240?) , exact same preamp, same power amp but with twice the transistors (8 in total), for 240W RMS into 4 ohms ... driving 2 x 15" Altec 418 or 421 ... a BEAST of an amp.

Impossible to google its schematic because current brand owners abuse it by naming "SVT" anything they make, even if the only tube it ever saw was a toothpaste one.

Oh well.

No need to say that it was a big heavy head, had a huge transformer and lots of heatsinking.

Hey!!!! now you have the perfect excuse to buy yet another cabinet

A single 15", preferrably an EVM15L would be a perfect match.