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 Post subject: Power output of KHL model 21?
PostPosted: Nov Fri 16, 2018 12:45 am 
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Location: Cortez, Colorado
Got a model 21 I've almost finished recapping. It seems to get loud enough, but distorts with the volume half way up. I measured 4 volts RMS at 200Hz, before clipping, that would give me about 2 watts into a 8 ohm speaker. The little speaker is really moving.


Last edited by Tin Omen on Nov Fri 16, 2018 1:47 am, edited 1 time in total.

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 Post subject: Re: Power output of KHL model 21?
PostPosted: Nov Fri 16, 2018 1:40 am 
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I'm a bit confused. Are you asking about the KLH 8 or the KLH 21?

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John Stone
Antique Radio Club of Illinois


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 Post subject: Re: Power output of KHL model 21?
PostPosted: Nov Fri 16, 2018 1:46 am 
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Location: Cortez, Colorado
Oh sorry, 21. I'll edit the above post.


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 Post subject: Re: Power output of KHL model 21?
PostPosted: Nov Fri 16, 2018 9:53 am 
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Tin Omen wrote:
Oh sorry, 21. I'll edit the above post.

Do you have the service manual for the Model Twenty-One? I have an original and have scanned it. I can send you a copy in about a week. I’m in Australia now but will be back in the US on 22 November. Just let me know; yours for the asking.

(Bug me if you ask for it and I forget)

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 Post subject: Re: Power output of KHL model 21?
PostPosted: Nov Fri 16, 2018 4:41 pm 
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Location: Cortez, Colorado
I have the service manual. I've checked the voltages, resistances and transistors. All the voltages seem a little bit low, my VCC starts at 21.6, instead of 22 that KLH shows. My voltages around the output transistors are more correct than the voltages that KLH lists.
The bottom transistor (Q334) seems to be doing most of the work. There is a large signal going into the base and about the same size going out the emitter, like it should be an emitter follower. And there is a tiny bit of signal loss through R328.The signal going to the top transistor's (Q333) base is much smaller, I guess because it's amplifying.
I'm getting some 15 volts of signal into the phase splitter transformer.


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 Post subject: Re: Power output of KHL model 21?
PostPosted: Nov Fri 16, 2018 10:12 pm 
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I have one that I think is working properly. I can measure the output power for you if that would help.

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John Stone
Antique Radio Club of Illinois


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 Post subject: Re: Power output of KHL model 21?
PostPosted: Nov Fri 16, 2018 10:43 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Cortez, Colorado
Yep, I used a signal generator, I got 12 volts peak to peak at the speaker terminals on an oscope, and on the fluke 87 it said 4 volts before distortion with 200Hz in.
With some FM station playing music I got somewhere around 3.5 volts RMS before I could hear distortion, but then I have it open and the speaker should be sealed. I guess the speaker connector on back would be just as good.

It seems like with their single-ended/push-pull circuit with 2 NPN transistors and a 21 volt power supply, that 2 watts might be all that I would get.


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 Post subject: Re: Power output of KHL model 21?
PostPosted: Nov Fri 16, 2018 11:30 pm 
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I'll check mine and get back with you.

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John Stone
Antique Radio Club of Illinois


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 Post subject: Re: Power output of KHL model 21?
PostPosted: Nov Fri 16, 2018 11:58 pm 
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Location: Sunnyvale CA
Tin Omen wrote:

It seems like with their single-ended/push-pull circuit with 2 NPN transistors and a 21 volt power supply, that 2 watts might be all that I would get.


It's certainly plausible that this is all it has.

Brett


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 Post subject: Re: Power output of KHL model 21?
PostPosted: Nov Sat 17, 2018 9:48 am 
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Brett_Buck wrote:
Tin Omen wrote:

It seems like with their single-ended/push-pull circuit with 2 NPN transistors and a 21 volt power supply, that 2 watts might be all that I would get.


It's certainly plausible that this is all it has.

Brett

I’m no engineer. But seat of the pants says that is about correct.

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Don


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 Post subject: Re: Power output of KHL model 21?
PostPosted: Nov Sat 17, 2018 11:19 pm 
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I just measured mine, and the output voltage into a 8 ohm load gives just under 4 volts rms @1kHz. The top of the waveform clips a bit sooner than the bottom, not unusual in an amplifier with this configuration. The dcr of the speaker is a little over 8 ohms, so the actual impedance of the speaker is likely closer to 10 ohms than 8. An 8 ohm speaker will usually have a dcr around 6 ohms. But I'd have to do an impedance curve measurement to confirm it. So it appears that your unit is pretty much operating as it should.

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John Stone
Antique Radio Club of Illinois


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 Post subject: Re: Power output of KHL model 21?
PostPosted: Nov Sun 18, 2018 12:36 am 
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Location: Cortez, Colorado
Thank you everyone. I lowered the output of the audio signal generator, and the volume control seems to be the linear type. After half way, it doesn't do much, and that's about where the distortion appears. So I'll wrap it up.


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