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Old 12th Jan 2021, 4:55 pm   #1
Avid_Nerdlinger
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Default Crossover distortion?

Got a Bambino-9 (9 what?) for 8.80 off ebay. It's all germaniums. Cleaned up nice. There were some dry joints and the black speaker/earphone wire was wrongly connected to the same node as the -ve battery... oops, that was probably me when I first got it a few months ago.

Reception is quite good, detachable telescopic aerial doesn't appear to make any difference.

Sounds very tinny and a bit clipped/distorted as if it has a broken speaker but it's the same on an earphone. Tried changing all electrolytics, no improvement. Tested with a tone modulated carrier. Nice sine waves at the bases of the output pair but that scope trace at the emitters. Also low and high tone frequencies lower the amplitude a lot.

Serial number 1001 suggests to me it might not have actually gone into production.

Is there likely to be a faulty component here or is this performance to be expected? Can that tinniness be improved by just dropping in a better (Si) replacement pair of output transistors?
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Old 12th Jan 2021, 5:07 pm   #2
Nuvistor
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Default Re: Crossover distortion?

Check the bias resistor network in the base of the output transistors and the emitter resistor.
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Old 12th Jan 2021, 5:14 pm   #3
Al (astral highway)
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Default Re: Crossover distortion?

Quote:
Originally Posted by Avid_Nerdlinger View Post

Is there likely to be a faulty component here or is this performance to be expected?

I can't comment on this specific set, but quite a few sets with a similar configuration have come my way over time.



Listening to one with any expectation of voice or musical fidelity may lead to frustration. They were never intended for music enjoyment, more for news and sport on the go. What the contemporary ear codes as 'tinniness' is pretty much built into them!
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Old 12th Jan 2021, 5:55 pm   #4
Craig Sawyers
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Default Re: Crossover distortion?

Well, the maximum power dissipation of the output transistors (2SB33) is 150mW, and the maximum collector current is 50mA. Which is exceptionally feeble, even for a germanium device...It is no wonder you are seeing distortion.

The good old Mullard OC81 was 600mW and 200mA (with a heat dissipator clipped on that is!).
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Old 12th Jan 2021, 6:45 pm   #5
Avid_Nerdlinger
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Default Re: Crossover distortion?

Thanks Nuvistor, one of the bias resistors on the bases is 3k3 on the schematic but 4k7 was fitted, so not pulling the base voltage far enough away from the emitter voltage to switch properly I think, I'll have more of a think about that, I should know this sort of thing, my imposter syndrome at work might have been imposter guilt. The emitter resistor looks OK.

Fitted a 3k3 and the emitter waveform looks better. I'll test it on radio stations tomorrow when the ionosphere gets burnt off...
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Old 12th Jan 2021, 9:13 pm   #6
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Default Re: Crossover distortion?

The speaker may also want several other offences to be taken into account.

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Old 12th Jan 2021, 9:19 pm   #7
Herald1360
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Default Re: Crossover distortion?

The tiny audio transformers won't do the quality any favours either- compare with what're fitted to a Bush TR82 for instance. The output transistors also need to be decently matched for best results.

Take a careful look at the output stage idle current (volts across the common emitter resistor)- too little = lots of crossover distortion, too much could lead to thermal runaway! If the set still sounds crappy (rather than just tinny) with a few mA, matching is probably poor.
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Old 14th Jan 2021, 3:13 pm   #8
Avid_Nerdlinger
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Default Re: Crossover distortion?

With clear reception today, at least on BBC R4 LW, audio still sounds a bit tinny but otherwise pretty good considering and no distortion now. Measured about 150mV across emitters resistor at full volume so the feeble output pair should be OKish. The speaker seems to have survived the offensive DC, I only realised the mistake (possible cause: drunk soldering) after flattening one set of batteries and noticing slight warmth from the speaker...

So unfortunately not HiFi, looks nice though, maybe even De-Luxe. Might put some lacquer on the detachable aerial which is growing some king of verdigris where the chrome plating has flaked off even though it doesn't help rx.
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Old 14th Jan 2021, 4:41 pm   #9
Andrew2
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Default Re: Crossover distortion?

It even has the cheek to proclaim 'Hi-Fi' on the front!
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