Laney BC30 lower power?

[GainFreak]

Hi,

A friend brought a Laney BC30 which looks like it has lower than specified output.
The power amp has signs of previous repairs. Some SMD transistors are changed to BC846 and BC856.
Measured 5.5mV and 7mV across 0.47 Ohm resistors which doesn't look like what is indicated in the schematic
Sine wave at the output looks OK but output power is only ~20Watts (9V and 13V at 4 and 8 Ohm loads at clipping).
Did Laney just overrated the output power and this is normal or else?

[Jon Snell]

If the output stage reaches 80% of the 50volts available, that is about right.
30Watts music power is about 18 - 20Watts ish, depending on expected distortion levels.

[GainFreak]

Should I worry about not getting those 40mV across 0.47 Ohm resistors as on the schematic?

[Jon Snell]

I wouldn't if it sounds OK, it's not HiFi after all.

[vintagekiki]

The quiescent current is defined with R18, D7 and D8 for typical h_FE values TS1, TS2, TS7, TS8 and is 85mA. In your case, that current is 12mA and 15mA.
Question
Does at the moment of clipping, DC voltages ⁺/_- 25V retain their value or drop.

[GainFreak]

At ~20W full power with minor clipping power supply goes from 25.7V down to 20.8V, ~317mV at 0.47Ohm resistors.
Replaced R18 with a trimpot and got 40mV at 116 Ohms. Neither sine wave nor output power were affected by this.

[Enzo]

Not sure what you are doing. Bias adjustment is done at idle, not with signal. Bias adjustment would affect crossover distortion, not output level or power.

[GainFreak]

Bias is 40mV at idle without signal but with trimpot at 116 Ohms not the stock 100 Ohms as already mentioned above. 317mV was measured at full power with minor clipping.

[Helmholtz]

I agree, the amp works as it should. No circuit manipulation or adjustment will give you more output power.

Rather max. output is limited solely by supply voltages.
You measured 20.8VDC at full output. Transistors and emitter resistors may keep 2V total for themselves.
That leaves around 19V for peak output voltage.or 13.3Vrms giving 22W into 8R.

[vintagekiki]

The change in quiescent voltage is occurred because you changed some SMD transistors, and by changing R18 you set the power amp to operating mode Uqv = 40mV

Drop DC voltage (from 25.7V down to 20.8V) may indicate that the main elco (C12 / C15) has changed capacity.

If at full power at the same time drop PT voltage (~ 36V), may indicates that the transformer for the predicted power reached required AC current.

When measuring sine output power use a resistive dummy load (4 or 8 Ohm), tone generator, oscilloscope ...

[GainFreak]

I decided that considering the vibrations (it's a bass combo) soldering and gluing a trimpot to those tiny SMD pads would be a hassle (few Ohms change the bias considerably) so I left the stock 100 Ohm resistor.
Thanks to all who chimed in.

[g1]

One thing about that bias info on the schematic, it's a little ambiguous, but I take it that one meter probe goes to each of the test points shown. So actual idle voltage would be 20mV across each emitter resistor.

Nowhere am I able to find any specs for the output power or how it is rated, they don't even show AC voltages for the power section on the schematic. So I think your ballpark numbers are close enough, unless the filter caps are worn out like vintagekiki suggested. If so, you may have more hum than normal.

[J M Fahey]

13VAC @ 8 ohm is fine, about what is to be expected there, 20W RMS is fine.

Dropping to 9VAC on 4 ohm shows that amp is not designed for 4 ohm load, period.

2 possible causes: either tiny power transformer is too tightly designed ($$$$$ you know) or short protection circuit is too "jealous" BUT I would not mess with it anyway.

A perfectly working 20W amp is miles a head of a dead 30W one, that if you actually get those 30W

Marketing dept has a lot of say about what is printed, not that much on the Tech side, FWIW I can point at another Laney example over rating stuff: they used to make an amp called the HCM25, HCM meaning Hard Core Max, a nice good sounding Guitar amp, based on a TDA2050 chipamp, rated 25W RMS well within chipamp capabilities.
Next year or two, they had to show "news" so the **exact same** HCM25 was renamed "something"30 , go figure.
Only change was front pan el design and label, and in fact service schematic for "****30" has still written inside HCM25.

HERE, I found it: both on brochures, manuals and visible labels, they call it the "1998 HCM30" but inside itīs still the "1996 HC25"

And Laney is a "serious" company, imagine others.

Mind you , this little bugger has killer sound and is quite loud for a 30W(ahem ... 25) W amp

FWIW I actually listened to both "30W" amps in "difficult" conditions, and they worked very well.

I live in Buenos AiresīLa Boca , an old and picturesque Tourist attraction today, since forever our riverside port, in fact Buenos Aires was founded in 1536 for the first time (Indians didnīt agree and wiped it out) just at the corner, besides the river.
Once a US Air Force band came to play there, go figure.
Lots of brass and drums, as expected, but they included electric Guitar and Bass.
Which were perfectly heard among the loud honky Music.
Not overpowering, not lost, same balanced level as others.
Obviously I rushed to check what they were using ... I was surprised at finding 2 little "30W" Laneys, each sporting a single 10" speaker.

[GainFreak]

So I think your ballpark numbers are close enough, unless the filter caps are worn out like vintagekiki suggested. If so, you may have more hum than normal.

I was thinking of replacing them but amp is dead quiet so I guess I don't have to.