Is my amp cap coupled?

I was given a Nikko 501 this weekend by a relative and by habit I always check the DC offset on amps before I hook them up. I measured nearly 9V of DC on the terminals and got a little worried. I decided to hook a pair of tester speakers and it sounded great, measured DC at the terminal with the speaker connected and it had dropped to about 13mv. Does this mean that the amp is capacitor coupled?

I have very little electrical knowledge so bear with me please. I read that a light bulb tester could be helpful so I soldered some speakerwire to light bulb and hook it up to the terminal but didn't see it light up in any way. If i'm totally off base here let me know. I also read that most CC amps will have 3 large filtering caps while DC coupled amps only have 2. The Nikko only has 2 large filtering caps from what I could see so i'm very confused. Any help would be much appreciated, I just don't want to hook up any nice speakers to this until i'm confident it's safe.
 
The Nikko STA501 is not capacitor coupled. It can't read 9V offset and be OK at the same time.
If both channels read the same could be meter error.

What's the resistance across each speaker output to ground? (turned off condition)
 
the STA-501 is direct-coupled from the outputs to the speaker, but the output stage is transformer-coupled to the stage before it (not sure if it's a driver stage or gain stage). i don't know how this would affect the voltages you measured, but it's certainly possible that it could explain it.
 
the STA-501 is direct-coupled from the outputs to the speaker, but the output stage is transformer-coupled to the stage before it (not sure if it's a driver stage or gain stage). i don't know how this would affect the voltages you measured, but it's certainly possible that it could explain it.

I don't think so.

Also, if both channels read the same, the output stage PS could have a problem. Digging into the equipment would be necessary.
 
Actually the design has zero DC feedback and actually can be OK like that. When you stick a load on it, all that offset was not all that much current. Voltage drops like a rock.

Little bit different kind of amp than everyone is used to actually.
 
Actually the design has zero DC feedback and actually can be OK like that. When you stick a load on it, all that offset was not all that much current. Voltage drops like a rock.

Little bit different kind of amp than everyone is used to actually.


It doesn't make sense to me.

So, how do you say the output stage is working to allow such DC offset?



This diagram is actually from another thread going on right now where some service voltage values are there to be checked. Pretty similar in design to Nikko's 501. Disregard the red circles; these are pointing out something else in the original thread.





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Actually the design has zero DC feedback and actually can be OK like that. When you stick a load on it, all that offset was not all that much current. Voltage drops like a rock.

Little bit different kind of amp than everyone is used to actually.

You may be correct, when I measured the DC offset with a test speaker connected my DMM only reads about 13mv. The sound playing through the test speaker sounds good, playback is strong. I was under the assumption that 8-9V on the terminals would damage speakers or at the very least not sound good at all.

Would there be any other relevant tests that someone with little knowledge and a multimeter could pull off to confirm this is safe for good speakers?
 
The Nikko STA501 is not capacitor coupled. It can't read 9V offset and be OK at the same time.
If both channels read the same could be meter error.

What's the resistance across each speaker output to ground? (turned off condition)

I'm sorry i'm very new to all this and have almost no electrical knowledge. Where should I place the probes to measure this resistance?
 
I'm sorry i'm very new to all this and have almost no electrical knowledge. Where should I place the probes to measure this resistance?

Right at the amplifier speaker output terminals; red meter probe to either L or R output, black meter probe to GND.

Remember to carry this out with the amp turned OFF.
 
I have an amp from this line, based on the same principle (transformer coupling before the output stage). Frankly I don't remember how the DC offset measured, but I can take it out and check when I have a minute.

By the way, it is an excellent amp, with very clean and detailed sound, no high tones roll-off, nice punch and bass control, and the only issue are noisy transistors in the tone amp. In TRM-1200 tone amp can be switched off, so I didn't even bother with replacing them.
I would not swap it for a Technics, even if Technics had much better specs. And it does not use regular transformers for coupling, they are trifilar coil transformers that don't have the disadvantages of conventional transformers in power amp circuit (they have wide frequency response and minimum distortion). At least that what Nikko claimed in their materials and, judging by the sound of the amp, it does work.
 
Right at the amplifier speaker output terminals; red meter probe to either L or R output, black meter probe to GND.

Remember to carry this out with the amp turned OFF.

I had the meter on the "200" setting for resistance, and I got a reading of about 175 immediately which slowly rose to about 197 before stopping.
 
I just noticed that if I set the selector to an input with no signal and turn the volume all the way up the woofers pulse in and out slightly at an even rate. This pulsing is far more pronounced on FM pulling in a station, at about 25% on the volume knob the woofer pulsing is clearly noticeable. The rumble filter switch will stop this from happening but what is most odd to me is that once the rumble filter is turned back off the pulsing doesn't resume until 5-10 seconds later. I can flip the rumble switch back on and off and replicate this effect, every time the switch is flipped on then off again the pulsing is gone for about 5-10 seconds before resuming.
 
Could be the noisy transistors I mentioned. Or a bad cap somewhere. Whatever it is, it's just the amp's noise floor that you amplify to the max by turning the volume knob all the way up. And it's in the preamp section, otherwise rumble filter and volume pot would have no effect on it.

I took out my Nikko and measured DC offset on outputs. It's nowhere near your measurement, without load it fluctuates but never goes above 100 mV, most of the time it's 20-60 mV. Looks like there is a problem with your 501, but maybe another AKer who owns a 501 or 701 can measure his. TRM-1200 is more powerful and uses much bigger coupling transformers, so it could measure differently for a number of reasons.
 
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Could be the noisy transistors I mentioned. Or a bad cap somewhere. Whatever it is, it's just the amp's noise floor that you amplify to the max by turning the volume knob all the way up. And it's in the preamp section, otherwise rumble filter and volume pot would have no effect on it.

I took out my Nikko and measured DC offset on outputs. It's nowhere near your measurement, without load it fluctuates but never goes above 100 mV, most of the time it's 20-60 mV. Looks like there is a problem with your 501, but maybe another AKer who owns a 501 or 701 can measure his. TRM-1200 is more powerful and uses much bigger coupling transformers, so it could measure differently for a number of reasons.

Thanks for the insight and running the test on your amp.

The woofer pulsing seems to be an issue on inputs with a signal, at normal listening volumes the pulsing is just as pronounced as it was on aux with no signal at full volume. That's why i'm a little worried about it, if it only happened at full volume it wouldn't be an issue.

As for the offset I noticed that if I measure from terminals with no speakers I see 9v but if I measure at the terminals with a speaker connected to them I only see 13mv. Does this mean the voltage is no longer present with a resistive load like some have suggested?
 
The cold resistance values you measured make sense.

I suspect that the issue is in the power supply. You'd need to measure the rail voltages (+/-23.5V). If there is a problem with the filter capacitors (mostly ageing) you'll find it with this measurement.

If you would like to carry this out let us know and we'll try to walk you through.

The speaker cone movement you see is low freq oscillation. Could have several sources, including ill PS.
 
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The cold resistance values you measured make sense.

I suspect that the issue is in the power supply. You'd need to measure the rail voltages (+/-23.5V). If there is a problem with the filter capacitors (mostly ageing) you'll find it with this measurement.

If you would like to carry this out let us know and we'll try to walk you through.

The speaker cone movement you see is low freq oscillation. Could have several sources, including ill PS.

Yes please that would be fantastic if you could walk me through these tests. The only tech available in my town wants $100 just to take a look at it. If this test is dangerous in any way please walk me through any safety precautions or procedures as well for working with the equipment if you don't mind. I'm still very new and don't want to hurt myself or the equipment.
 
The voltages inside the unit are not dangerous at all, except for the AC line input before the transformer.

You need to measure DC voltage on the 2 electrolytic main filter capacitors in the PS with the device ON. Select 200VDC in your meter and put the red probe on the spots pointed out with arrows in the image. The black meter probe should touch the bridge running between the 2 capacitors.

Be very careful handling the meter probes to avoid slips and shortcircuits!


Nominal value is +/+23.5V. Let us know what your read.
 

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