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Old 3rd Aug 2019, 9:12 pm   #21
Station X
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Default Re: Ineffective tone control.

Update.

Noticed that there's no VDR across the primary of the O/P transformer unless it's internal to the transformer, which I doubt.

Changed all the resistors with the exception of the 680R in the smoothing circuit which was within spec. While the 680k was out of circuit I measured the parallel 100 pF cap at 105pF, so nothing wrong there.

Used 0.047uF and 0.01uF caps in parallel to replace the tone control cap. Specified value is 0.056uF.

New voltage readings.

HT at O/P TX 96.1V (Spec 90V)

Input transistor.
C 0V
B 2.85V
E 3.52V

Output Transistor.
C 93V (Spec 85V)
B 3.52V
E 3V (Spec 5.5V)

Despite the work done there's no detectable difference to the sound which is good by record player standards.

The tone control is still totally ineffective and might as well not be there. I've checked all its connections again just in case.

Tomorrow I'll plot the frequency response.
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Old 3rd Aug 2019, 10:47 pm   #22
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Default Re: Ineffective tone control.

Quote:
Originally Posted by Herald1360 View Post
Quote:
Originally Posted by Argus25 View Post

I think there is a missing DC negative feedback resistor from the emitter of the output transistor to the base of the input transistor, and if there really isn't one, there should be.

That would be positive feedback- both stages are emitter followers in that configuration.

This circuit is deceptively simple!
oops yes , sorry.
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Old 3rd Aug 2019, 11:10 pm   #23
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Default Re: Ineffective tone control.

Quote:
Originally Posted by Station X View Post
Update.


New voltage readings.

HT at O/P TX 96.1V (Spec 90V)

Input transistor.
C 0V
B 2.85V
E 3.52V

Output Transistor.
C 93V (Spec 85V)
B 3.52V
E 3V (Spec 5.5V)


The tone control is still totally ineffective and might as well not be there. I've checked all its connections again just in case.

Tomorrow I'll plot the frequency response.
With the correct DC conditions the collector current of the output transistor is about 46mA, and the DC resistance of the transformer primary about 109 Ohms (5V drop at that current). You test data shows about a 3/5.5 reduction in collector current to about 30mA, giving about a 3V drop across the transformer primary that you are getting.

The reason for this is the circuit design for its DC bias conditions is too dependent on the hfe value of the input transistor. If you were to increase the value of the 390k, you could adjust it to the specs. (although it would be worthwhile checking that the input transistor did not have any collector to base leakage, unlikely)

Also, lets say approaching full output the collector voltage swung down to 10V above ground, that would be 75V peak or roughly 50V rms. Lets say at max output it was putting out 4 watts, then 50^2/4 =625 Ohms, so the collector load impedance would be in that range.

For a tone control the pot value required would more likely be in the range of of a few k and the capacitor with a reactance of say 500 Ohms at 5kHz about 0.06uF. So perhaps its supposed to be a 5k pot and not a 50k.

So it appears the thing to do would be to fit a 5k pot (assume the diagram should be 5k) and increase the 390k until you get the 5.5V across the emitter resistor of the output transistor.

Last edited by Argus25; 3rd Aug 2019 at 11:17 pm.
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Old 4th Aug 2019, 5:02 pm   #24
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Default Re: Ineffective tone control.

The tone control does actually work. If a single frequency tone is injected at the cartridge leads the volume can clearly be heard to increase as the tone control is turned clockwise and vice versa. The same applies when listening to tones from a test record.

The same effect can be demonstrated by injecting tones at a fixed level and measuring the voltage across the loud speaker using a True RMS Voltmeter.

The problem is that, to me at least, the tone control seems to make hardly any difference to the sound when actully playing a record. I have tried with all sorts of records, pop, classical, speech etc.

I'll have to put this one down to experience!

All I have to do now is try to reduce the mains hum being picked up from the motor and mains transformer. The smoothing caps are fine.
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Old 6th Aug 2019, 9:21 am   #25
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Default Re: Ineffective tone control.

At which frequency to you start to hear the effect of the tone control? Could it be so high that you're just touching the top of the treble range?

Interesting amplifier design. I wonder if it was originally designed to be power-transformer-less operating from a 110V mains, that would explain the high voltage used. Otherwise there would be little point in operating a transistor amplifier at such a high voltage wouldn't there?
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Old 6th Aug 2019, 10:19 am   #26
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Default Re: Ineffective tone control.

I suggest the circuit was probably a Motorola design as they supplied many data sheets with examples in order to sell their transistors. I used to have a large folder full where I worked in the late 60’s. Why design your own when they give you a ready made circuit. They probably supplied devices with suitable characteristics for the design.
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Old 6th Aug 2019, 10:51 am   #27
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Default Re: Ineffective tone control.

The MJE340 was used in the Thorn 8000 series TV chassis, same function.

Lawrence.
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Old 6th Aug 2019, 11:06 am   #28
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Default Re: Ineffective tone control.

My understanding is a transistor is a current amplifier and therefore has a high output impedance. The impedance that determines the cutoff of the tone control is therefore determined by the reflected impedance of the speaker.

Has the output transformer or speaker been changed?
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Old 6th Aug 2019, 12:29 pm   #29
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Default Re: Ineffective tone control.

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My understanding is a transistor is a current amplifier and therefore has a high output impedance. The impedance that determines the cutoff of the tone control is therefore determined by the reflected impedance of the speaker.
That is correct, and the impedance would be easily estimated from the speaker and the impedance ratio of the transformer. I estimated that if it was able to supply around 4W power just before clipping, the impedance would be in the order of about 600 Ohms as a rough guess. So the 50k tone pot value seemed awfully high, but the capacitor value there about right.

I think the output transformer is likely original because the DC voltage drop across its primary is about right for the DC current passing through it.
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Old 6th Aug 2019, 12:39 pm   #30
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Default Re: Ineffective tone control.

Back at the amplifier itself, it seems barely adequate unless a very high output cartridge is used- simulation suggests at best a hundred mW or so at the speaker for a 1V rms output from a simulated cartridge (voltage source in series with 900pF). My OP TX model is 25:1 giving about 2k load for the MJE340. This load was derived from a few assumptions about the p-p voltage and current swings available at the MJE collector.

It would be interesting to know what ratio is actually used if the OP's test gear (and patience!) is up to the challenge.

For a 2k load, to get a useful treble cut from the tone control circuit, you'd need about 100n cap and a 5k (log?) pot.
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Old 6th Aug 2019, 1:28 pm   #31
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Default Re: Ineffective tone control.

Quote:
Originally Posted by Herald1360 View Post
Back at the amplifier itself, it seems barely adequate unless a very high output cartridge is used- simulation suggests at best a hundred mW or so at the speaker for a 1V rms output from a simulated cartridge (voltage source in series with 900pF). My OP TX model is 25:1 giving about 2k load for the MJE340.
What a gutless wonder, looking at the photos Lawrence posted the output transformer looks like about a 3W core, so maybe it only cranks out 2W max, in that case I estimate the collector load impedance to be about 1250 Ohms and if it only cranks out 1W max about 2500 Ohms agreeing with your simulation.

The MJE340 is on a good heat sink, so with enough drive it could easily put out 5 or more watts, but the drive looks barely adequate as you say.

Another way to very roughly estimate the output power in a class A amplifier like this is to assume the wasted power from the standing current is about roughly equal to what the amplifier can deliver at full output, so 85V x 46mA = 3.9W, that of course being if there is enough drive power available to achieve that which looks unlikely.

Last edited by Argus25; 6th Aug 2019 at 1:45 pm. Reason: typo
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Old 6th Aug 2019, 4:19 pm   #32
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Default Re: Ineffective tone control.

I think we should remember that this amp is in a Fidelity unit that would have been low cost and used as a transistorised version of a 1 valve record player. The cartridge is probably a high output BSR.
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Old 6th Aug 2019, 9:41 pm   #33
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Default Re: Ineffective tone control.

Well said vidjoman. On paper and in simulation this may be a barely adequate gutless wonder, but as a Dansette style record player it works very well. There's plenty of audio power for a teenager's bedroom and the bass response is very good.

At the moment I'm giving it a long soak test by working through my LP collection.

If I have time at the weekend I'll open it up again and determine the turns ratio of the O/P TX to satisfy members' curiosity.
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Old 11th Aug 2019, 3:28 pm   #34
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Default Re: Ineffective tone control.

First results typed up while I have a coffee break.

Peak undistorted power is 1.7W, achieved between 8 and 11 kHz. This is with volume and tone controls at max and an input signal of 353mV RMS at the cartridge connections.

---------------------

With a 10kHz input signal the input level was adjusted to give 6V p-p across the speaker. About 1.5W

It wasn't possible to measure the voltage across the primary winding, as the earth from the 'scope probe would have shorted the HT supply.

The voltage from chassis to the OP transistor collector was 170V p-p.

I'm going to do a freq run with the amp feeding a dummy load/power meter over the range 1 to 20 kHz with the tone control set to min and max.

If simulators need more info please speak up now as I want to box up the player for the last time today.
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Old 11th Aug 2019, 5:15 pm   #35
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Default Re: Ineffective tone control.

For the freq run I decided to use a True RMS Voltmeter connected across the speaker to measure the power output. This would take into account the impedance of the speaker, which would vary with frequency, unlike a dummy load.

The input at 10kHz was adjusted to give 2V across the speaker with the tone control at max.

Frequency in kHz. Level with tone control at min. Level with tone control at max.
1 0.65V 0.68V
2 0.64V 0.92V
3 0.55V 1.30V
4 0.48V 1.63V
5 0.42V 1.90V
6 0.38V 2.05V
7 0.34V 2.11V
8 0.31V 2.10V
9 0.29V 2.05V
10 0.26V 2.00V
11 0.22V 1.90V
12 0.21V 1.90V
13 0.21V 1.85V
14 0.20V 1.82V
15 0.19V 1.80V
16 0.18V 1.78V
17 0.17V 1.75V
18 0.16V 1.73V
19 0.15V 1.71V
20 0.14V 1.70V
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Old 11th Aug 2019, 6:10 pm   #36
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Default Re: Ineffective tone control.

It seems to be doing something then, I've not entered all those into a calculator but from 5kHz to 16kHz approx. -17.5dB average in voltage terms if I've got it right.

Lawrence.
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Old 11th Aug 2019, 7:22 pm   #37
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Default Re: Ineffective tone control.

I could not resist plotting the frequency run and printing with both linear and log scales.

David
Attached Files
File Type: pdf Fidelity Tone Control.pdf (390.0 KB, 64 views)
File Type: pdf Fidelity Log.pdf (388.1 KB, 77 views)
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Old 12th Aug 2019, 8:35 am   #38
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Default Re: Ineffective tone control.

I guessed someone would do that. Thanks.

There's an odd dip in the response at around 11kHz, which caused me to recheck my readings. They are correct though.

Of course the tests only relate to the amplifier and don't take into account the frequency response of the cartridge. I don't have a suitable test record to check that.
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Old 12th Aug 2019, 9:13 am   #39
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Default Re: Ineffective tone control.

Just caught up with this..... the voltage readings suggest about 28:1 for OP TX which is near enough to the 25:1 I ended up with for a 2k load. I'll have another play and see what happens.....
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Old 12th Aug 2019, 6:09 pm   #40
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Default Re: Ineffective tone control.

Considering the sad reality that we all lose top end as we get, eh, more mature, have you considered getting a young whippersnapper to listen and say whether they detect any change?
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