Akai GMC-570D as headphone amplifier - advice on how to test?

[BadAudioAdvice]

Hello!

I consider myself incredibly lucky to have found a nearly mint physical condition, and in what appears to be good working condition Akai GMC-570D stereo cassette deck.

I absolutely love the looks of it and have it displayed in the living room next to the sofa.

I thought it would be convenient for easy listening since I would be able to plug headphones directly into the unit - however, I quickly discovered there was a major shortcoming...no volume control for the headphones! I was able to address that with a Rolls HV6 passive in-line volume control.

Now with the volume control sorted out, wouldn't it be nice to use it to listen to more than just cassettes, since I can also feed one set of RCA into it.

Got me wondering - how could I go about measuring whether the headphone amplifier in the Akai is any good?

In terms of equipment that I have access to:
- Behringer UCA202 USB DAC/ADC 48khz 16bit (note: online it is sometimes confused with the model that has the phono amp built-in)
- Digital oscilloscope
- A bag of resistors and a breadboard if I need to create a dummy load

Any advice would be much appreciated!

 

[Doodski]

3 head, touch control, meters and perhaps the motor(s) spin up on power ON too. Don't know although the lamps are certainly burning away their life expectancy. I would not leave this one powered ON all the time unless you anticipate some service in 2-3 years perhaps. It's a nice unit. Have you sourced belts?

 

[BadAudioAdvice]

The touch controls are fantastic. Very sensitive, consistent, and instant visual feedback. Just a pleasure to use.

The meters are very responsive.

Agreed, won't be leaving this one on unless actively listening.

Good idea, getting belts in anticipation of servicing.

At the moment everything is working well, enjoying listening to it as I type this.

 

[solderdude]

The headphone output is an afterthought.

It is the line output run through a very low power emitter follower which is connected via a small audio transformer as used in portable radios of that day. They used that same signal for the meters and just tapped it off for the headphone out.
Measured performance will not be great.

service manual

 

[BadAudioAdvice]

Thank you for the link to the service manual!

How were you able to identify the part used? I tried Googling' the part# from the manual but wasn't able to find anything...

 

[BadAudioAdvice]

To bump this thread, even though the headphone amp in the cassette deck isn't that great...I would still like to test it if possible.

Any advice on simple tests would be much appreciated!

 

[charleski]

You can use the RCA inputs for test signals from your UCA202. You need to rig up a dummy load cable that goes headphone plug->load resistors (choose a value close to the impedance of your headphones) soldered between signal and ground->output plugs to connect to the UCA202 input (this will be in parallel to the load resistors).

Free software to generate the test signals and analyse the results:
REW
RMAA

You can use REW to measure the output power of the headphone amp. You need a multimeter to measure the output voltage across the load resistors, use the Calibrate level dialog in the signal generator, measure across the load resistor and enter the value given into the FS sine Vrms field in the RTA window. You need to enter the load impedance (resistor value) into the Ref resistance for dBW field in the Appearance tab of the settings in the RTA window. Then run a stepped sine test and set the x-axis to dBW.

 

[DVDdoug]

Typically the "weakness" with headphone outputs is the output impedance. The source impedance along with the headphone impedance makes a voltage divider. And since headphone impedance varies with frequency that can mess-up frequency response.

You can check that by measuring the no-load output voltage (with a test tone), then add a load resistor (32-Ohms is a "typical" headphone impedance, so probably something close to that). Or, you can just plug-in your headphones. If the voltage doesn't drop, you're good-to-go. If the voltage drops you can calculate the source impedance.

You can generate test-tones with Audacity and you can also use those to check frequency response.

You could also check the noise and the maximum output-voltage before clipping, but your ears will tell you if you're hearing noise or if you can't get enough loudness without distortion.