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SOLID STATE AMP IMPEDANCE QUESTION

Davidr

New Member
The attached shows an 8 ohm load on a solid state amp, but in reality the amp is fitted with a 4 ohm load and has a broken (badly distorting) power amp stage. I'm planning to replace both power transistors (BD237 and BD238) and wondered if anyone can advise on the load. Most solid state amps can take loads of 4, 8 or 16 ohms. How can I tell if a 4 ohm load is safe on this when the circuit diagram shows an 8 or 16 ohm load ? Thanks.
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
The attached shows an 8 ohm load on a solid state amp, but in reality the amp is fitted with a 4 ohm load and has a broken (badly distorting) power amp stage. I'm planning to replace both power transistors (BD237 and BD238) and wondered if anyone can advise on the load. Most solid state amps can take loads of 4, 8 or 16 ohms. How can I tell if a 4 ohm load is safe on this when the circuit diagram shows an 8 or 16 ohm load ? Thanks.
Your statement is completely incorrect - only small numbers of transistor amps can feed as low as 4 ohms, most are 8 ohms or higher (there's no higher limit).

If the spec only says 8-16 then you mustn't go lower than 8 ohms, or you'll probably kill the amp. An amplifier that will feed 4 ohms will be clearly marked as such.

Basically halving the speaker impedance doubles the power - so if the amp is 100W to 8 ohms, then fitting a 4 ohm speaker it will try and output 200W and destroy itself.

Think of it as a car - it's maximum speed is 100mph - you fit some wrong component, it will now try and go 200mph, do you think the engine will survive?.
 

Davidr

New Member
OK thanks Nigel. I read the 4 ohm statement online and wanted to check it out. I don't have a spec just the circuit diagram. I have been given the amp from the son of a now deceased musician who says its been like that for years and his father never tried to repair it. The 4 ohm load has probably knackered the power amp - the pre amp is fine. I can only guess that someone has replaced the speakers at some point as its a combo amp with 2 x 10" speakers which are each 8 ohm, wired in parallel not series. Its not possible to get an 8 ohm load unless you only use one speaker, so my guess is that the speakers were replaced or perhaps miswired to try and get it louder.
 

crutschow

Well-Known Member
Most Helpful Member
As Nigel stated, you should not go lower than 8 ohms with that amp.
That will give the most volume without damaging the amp.

If you wanted to drive more than one speaker (which will not increase the volume but may reduce distortion for a given volume) you could use two 4 ohm speakers in series, or two 16 ohm speakers in parallel.
 

Davidr

New Member
Thanks. I know I'm a newbie but have been looking after valve amps for some time and so know about matching loads to what's required. I just wasn't sure about solid state amps.
 

crutschow

Well-Known Member
Most Helpful Member
have been looking after valve amps for some time and so know about matching loads to what's required.
Tube amps have a high impedance (for example a push-pull KT-88 pair has about 4.5k ohm plate-to-plate impedance).
The load is matched to this impedance for maximum power output (maximum power transfer theorem) by the audio output transformer (typically with 4, 8, and 16 ohm taps).
(I have seen designs where they put many output tubes in parallel to reduce the output impedance that supposedly eliminate the requirement for the transformer).

In contrast, solid-state amps have a very low output impedance so the output load is not matched for maximum power as that would cause a very high output current and damage the amp.
They deliver maximum power (without internal damage) to their minimum impedance rated load.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks. I know I'm a newbie but have been looking after valve amps for some time and so know about matching loads to what's required. I just wasn't sure about solid state amps.
Tube (valve) amplifiers use impedance matching, for maximum power transfer - whichis highly inefficient (less than 50%). Transistor amps use voltage transfer, which is massively more efficient than valve amps - so a VERY low impedance output (such as 0.01 ohms) feeding a much higher impedance load (4 ohms and above).

This is why valve amps can be damaged by disconnecting the speaker, while transistor amps can be damaged by shorting the speaker out (and have no issue at all with no load, in fact the higher impedance the load the safer the amplifier is).
 

crutschow

Well-Known Member
Most Helpful Member
valve amps can be damaged by disconnecting the speaker,
The reason for that is that with no load, the output transformer looks like a high impedance inductor, which can induce sufficiently high primary voltages from the tubes, if a signal is applied, to damage the transformer winding insulation.
 

Ylli

Active Member
Back to your original amp - I would suggest just wiring your 2 existing 8 ohm speakers in series instead of in parallel. The amp will then see 16 ohms, which will make the amp much happier. The max volume will be a bit less but really not that much less. If you are going to replace the output transistors, you might want to also consider replacing the drivers (TR7, TR8) as output transistor failure can often cascade to the drivers.
 

crutschow

Well-Known Member
Most Helpful Member
The max volume will be a bit less
About 3dB less compared to an 8 ohm speaker or 6dB less compared to the parallel configuration.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Solid state amps can be tested without a load. e.g. with a scope.

Check those small valued resistors (100 ohm in your case).

Something ends up limiting the amplifier. It could be the amount of current it can produce, the speaker Z or the power supply rails.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The reason for that is that with no load, the output transformer looks like a high impedance inductor, which can induce sufficiently high primary voltages from the tubes, if a signal is applied, to damage the transformer winding insulation.
I know, I didn't feel any need to explain it :D
 

Nigel Goodwin

Super Moderator
Most Helpful Member
About 3dB less compared to an 8 ohm speaker or 6dB less compared to the parallel configuration.
There's a LOT of crap about dB's :D

I can tell you now, the differance between two 8 ohms in series and two in parallel is absolutely massive - regardless of what the theory might infer.

Years back we had a pair of HH 2x12 PA speakers, wired in series, 16 ohms - we actually built an 'industrial' stereo for an artist friend (think flight cases) and sold him the 2x12's as the speakers. We rewired the speakers to be 4 ohm (as the amplifier modules were 4 ohm) and the volume difference was absolutely incredible. Obviously the actual power was then 100W per channel as opposed to only 25W per channel.
 

Tony Stewart

Well-Known Member
Most Helpful Member
The attached shows an 8 ohm load on a solid state amp, but in reality the amp is fitted with a 4 ohm load and has a broken (badly distorting) power amp stage. I'm planning to replace both power transistors (BD237 and BD238) and wondered if anyone can advise on the load. Most solid state amps can take loads of 4, 8 or 16 ohms. How can I tell if a 4 ohm load is safe on this when the circuit diagram shows an 8 or 16 ohm load ? Thanks.
It's not a bad design but the BD23x transistors are too low hFE =25 at 2A and why they are obsolete. I would replace the 3 diodes with a "diode multiplier" cct to bias the output stage at 20mA to avoid crossover distortion at high current and thermally match that with the power transistors.

Speaking of which these transistors have hFE=120~240 which at a minimum is 120/25=~5x the current gain which will easily handle much lower impedance speakers.

https://www.el-component.com/bipolar-transistors/ttc3710b 1580916519665.png1580916519665.png
 

audioguru

Well-Known Member
Most Helpful Member
Speakers in series sound boomy because they do not have the extremely low output impedance of a solid state amplifier damping the resonance.
Also the 40 ohms resonance of each speaker is is series reducing the bass output level.
 

unclejed613

Well-Known Member
Most Helpful Member
you have 1A fuses specified in the schematic, and a rail voltage of +/- 29V... 29V rails are about right for 50W into an 8 ohm load. if you clip the amp to the point where the output exceeds 60W, you will blow the power supply fuses. this amp should be run with an 8 or 16 ohm load.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Speakers in series sound boomy because they do not have the extremely low output impedance of a solid state amplifier damping the resonance.
Except they don't, they might in theory, but in practice HUGE numbers of professional speakers and sound systems are wired in series with no such issues.
 

Ylli

Active Member
Except they don't, they might in theory, but in practice HUGE numbers of professional speakers and sound systems are wired in series with no such issues.
I think it might also depend on how they are acoustically coupled. If they are in the same box, they may act more like a single speaker than if they are in separate boxes.
 

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