[TaffyGuy]

well i've moved to school that has just started which means my DIYing and even listening have been on hold recently but now that i'm sitting here fiddling in the middle of the night, I can't help but notice my 8620 meta works on 6 volts. 5 volts is definately out of the question, but it works just fine at 6. works fine at 22 as well, and i havn't gone above that.

I have come upon these marvelous revelations due to the generous contribution of an HP voltage/current supply from my retired EE father, which I am still using as a PS for my meta because i'm too lazy to go deal with wall warts and dinn sockets or whatnot. soon though.

anyway, the short of it is that it works at 6 and i'm not sure what makes the optimal voltage 20-22, since it doesn't "seem" to change. of course around here things aren't always as they seem. haven't broken out the DMM since i assume that this very nice piece of vintage HP lab equipment is functional and accurate, and have no reason to think otherwise. sure doesn't work at 5 volts. perhaps it would start to misbehave if i turned up the volume. doesn't seem like it.

so whats the deal?

 

[ppl]

This seams fine to me as my tests with the AD-8620 & EL-2001' indicate +/- 3 Volts will run the Amp just fine but output Voltage will be Reduced alot vs +/- 9 Volt Operation

 

[KurtW]

This reminds me, I took some THD measurements verses power supply voltage a while back and I forgot to post it. The measurements were made at 1 kHz with some 32 ohm Philips HP550 headphones as a load, at what I would consider a normal volume. This is for a META42 with an AD8620 and single EL2001 buffers. I used a variable power supply feeding the META which is set up for batteries. Here's a summary of the data:

PS voltage, % THD
5.0, 0.75
6.0, .57
7.0, .43
8.0, .42
9.0, .41
10, .40
12, .39
14, .37
16, .36
18, .35
20, .33
22, .32
24, .31

I usually use this amp at 10 volts, so I was happy to see that the data seems to agree with my opinion that it sounds almost as good at 10 volts as it does at higher supplies (assuming you aren't limited by the output voltage of course). And it does show it's not bad at 6 volts either, as ppl and TaffyGuy indicate.

 

[ppl]

KurtW;
Thanks for the info. However your THD seams high the EL-2001 will do 0.75% THD @1KHz. & 100 Ohms all on it's ownwithout the aid of feedback. However Your numbers are what one can expect in real world operating Conditions. using Real headphones as the load will Raise the THD alot vs using standard load resistors. Also one EL-2001 is not recomended for loads less than 100 ohms IMHO. for that load use 4 in parr. and at least 2 as VGD with lots of rail cap. I get 0.09% to 0.1% THD at 1KHz. & 1 Volt RMS into 100 ohms with +/- 9.2 Volt rails. this is with the Feedback set up for about 30 dB of overall feedback around the The Entire circuit.

 

[promptcritical]

KurtW,

How did you make the THD measurements?

Please feed measurement junkies like me!

Thanx

 

[tangent]

That's not too surprising, TaffyGuy. The 8610 datasheet says that at 300 ohms, the output to supply rail difference will be around 1.5V, so that leaves about 3V peak-to-peak for the output with a 6V supply. That's enough to drive many headphones to quite high volume levels. As the load impedance goes down, the output-to-rail distance will go up, reducing possible output voltage, but low-impedance headphones tend to need less voltage anyway.

On my toughest test, this chip wants up to 10V to get rid of the last of the clipping and easily audible distortion, but real life is a lot easier than my torture test.

For what it's worth, you can get down to about 3 or 4V in a META42 type amp if you use an RRIO type op-amp like the AD823.

 

[Jupiter]

What will a META42 with high THD values sound like? Distorted or bright?

 

[KurtW]

Your right ppl, load makes a big difference. I no longer have the amp I originally measured but I measured two others, both with AD8620s (thanks ppl), double 2001 buffers (thanks again) running from 9 volt batteries (one measured 8.9v, the other 9.8v, athough the distortion levels at 1 v output were the same). At 1 volt output at 1kHz, I measured the same as you, 0.10% THD with a 100 ohm load. Change the load to 31 ohms like many headphones, and I get 0.19%. Plug in the Philips headphones, which are 32 ohms nominal, and I was amazed that I got the same thing. At 1 volt the tone from the headphone was very loud, I'm glad I didn't have it on. At lower voltages the distortion actually goes up, .29% at 0.25v, which is closer to what I had measured before. With either 100 or 31 ohm load, the distortion actually drops slightly at higher voltages, 0.09% at 1.5v with 100 ohms. Go to 1.6v though, and it shoots up to 4.3%! If you need this kind of output voltage, you need more than a 9v battery.

By the way ppl, my amps are small cases...one is in an Altoids tin. I can imagine stacking 4 buffers and needing to cut a hole in the top of the lid...it would look like a 60's muscle car!. Three would fit though.

Jupiter, you probably wouldn't hear much difference between 0.1 and 0.2% distortion, but when it suddenly runs out of gas and jumps way up, it sounds very distorted.

promptcritical, I use a Neutrik MR1 generator and ML1 analyzer. By the way, the analyzer also does 1/3 octave spectrum analysis. I noticed that there was more odd harmonics than even in the distortion products. For 1kHz input, 2kHz was down -65db, 3k down -62dB, 5k down -68dB, the rest were much lower down.

 

[ppl]

KurtW> Odd harmonics are standard with Complimentry Topologies. To lower this as well as Other THD products More Negitive Feedback. Also the ratio of Resistor values in the Outside loop and To the Inside loop matter alot I use 4.7K & 1K in my pocket amp with these buffers and 2 Meg & 4.7K around the Opamp. You can evean go up to 10K & 4 Megs around the Opamp For slightly Better results.