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Power Amp headphone adapter?

post #1 of 11
11/27/11 at 5:38pm
Thread Starter 
I just picked up a Parasound P/SP1500AV and HCA750A combo. The problem is (actually there are 3, two of which I new about) that the pre has no HP out.

The ESP P100 project looks to be what I need to do here, but everything I have seen says 120Ω output is not good, closer to 0Ω is and a high DF is good here. So, my question is, how can I modify this to output useable voltages at a very low Z-Out? I have no idea what to assume for my amp's values to mess around with this for modification, all I know is 75W into 8Ω loads. I've tried contacting ESP to find the values used in his specs, but I have not received a response there and don't have a way to test my values right now to do everything on my own here.

So, if anybody can help modify this circuit into something with a lower Z-Out I'd really appreciate it. I'd rather see how my headphones respond to amping from this than drop a bunch of money on a dedicated amp and hear nothing of interest in improvement (not so worried about the V6, future cans are the concern).

And if anybody has used this preamp, I have another question about that HERE
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post #2 of 11
11/27/11 at 11:26pm
Thread Starter 
Anybody? I know I can't be the only one that wants to try this out.
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post #3 of 11
11/28/11 at 12:27am

Its a very simple and cheap project, why not just build it and see how it sounds with your headphones?

 

 

 

 

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post #4 of 11
11/28/11 at 12:38am

Applied Research & Technology: ARTcessories™

 

Have not yet had a chance to try this product (my stuff is out on loan and I need to make some cables).

 

Problem is that they list INPUT impedance as 150 ohms and not OUTPUT impedance.

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post #5 of 11
11/28/11 at 1:30pm
Thread Starter 
Quote:
Originally Posted by Fred_fred2004 View Post

Its a very simple and cheap project, why not just build it and see how it sounds with your headphones?
It would be cheap if I didn't have to order every one of these resistors and pay shipping. RS carries up to 1W in select stores and they are cheap and I've had a few pop on me using them near the rated power. I would prefer to use a 1.5-2W resistor with the specs detailing R1 as having up to 1W across it. I could do a series/parallel network thing on the R1 area and use 4 1/2 W resistors at the rated value to achieve that goal, but it's not all that optimal for this project.

Regardless, the values seem off in this, running everything through a circuit simulator, I get that it takes 30V from the amp to drive a 600Ω load to 5V, but that consumes 1.3W at R1, not barely pushing 1W. Any lower headphone load raises the amp voltage and the R1 power considerably (8Ω is ~375V at 271W across R1) so I don't even know what load he is saying this thing should be able to put out 5V at. 600Ω is about as high as we ever go, and the closest to any of his values, but it's still high.

I'm planning on buying some heatsinked wirewound resistors and some isolation transformers for a ground loop problem and a dummy load box at some point, so I may have to do that now to save on shipping for this.

And CEE TEE, I saw a few of those, but the 1/4" input doesn't really work for my needs, and like you said, the input impedance is 150Ω.
Edited by sml1226 - 11/28/11 at 1:31pm
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post #6 of 11
11/28/11 at 2:03pm

I think you need to check your math, using the starting point that your speaker amp can put out 30V.


Edited by bcg27 - 11/28/11 at 2:04pm
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post #7 of 11
11/28/11 at 3:13pm
Thread Starter 
Quote:
Originally Posted by bcg27 View Post

I think you need to check your math, using the starting point that your speaker amp can put out 30V.

The thing is, I'm not sure what my amp can put out. Using a circuit sim to find something that meets the ESP values led me to 30V in and a 600Ω load to get something remotely close to the listed values. I may have, in fact, for anybody that wants to see the sim

HERE is the sim, and the circuit I have at this point to get similar values to his.

screenshot20111128at501.png


Everything could be completely wrong, but using his numbers and that sim, I'm getting that I'd need a 30V output and 600Ω cans to get his values which doesn't seem right, at least the need to use 600Ω considering the nature of the site in regards to headphones. All of his mentions of headphones say 8Ω and 32Ω which are not making sense as, like I said, at least with the sim, 8Ω requires an insane 375V and 270+W resistor in this scenario.

Also, on the "Hi-Fi Headphone Amp" project (P113) he again pushes for the 120Ω output, even after finding that listeners find the closer to 0Ω output "unfortunately" sounds better, as most headphone amp makers have decided to strive for anyway disregarding the "standard" 120Ω.
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post #8 of 11
11/28/11 at 3:49pm

Well that looks mostly correct except the website specifies a 560 ohm r1 for a 100 watt amp into 8 ohms (30 volts is 112.5 watts). That being said, as you have found out it will not be able to supply 5 volts into a low impedance load (although I am not aware of any 8 ohm headphones) as the amp will be current limited. However I think you will find that the vast majority of headphones don't need anything near 5 volts so that is not an issue IMO. BTW your amp is 75 watts into 8 ohms according to parasound's website so it can put out about 24.5 volts.

 

FYI, 3 watt vishay resistors are 46 cents a piece at mouser check out this page: http://www.mouser.com/Vishay/Passive-Components/Resistors/Film-Resistors/Metal-Film-Resistors-Through-Hole/_/N-7gz41Z1z0zls5Zscv7?P=1z0x6frZ1z0vo2a&FS=True

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post #9 of 11
11/28/11 at 4:23pm
Thread Starter 
I wasn't worried about supplying the specified voltage to anything, 5V is extremely high, I was just confused as to the headphone impedance being assumed here. He seems to know his stuff in speakers, but everything related to headphones has been counter-intuitive and contradictory (namely the 8Ω output specs for a headphone amp). I feel dumb for not being able to figure out my voltage now though... never thought of using the P=V^2/R formula for that...

Anyway, that seems to be the best bet until I can figure out a design that will accomplish the attenuation without the impedance being as high as it is.

Thanks, I still can't figure out his values for headphone loads (best I can get at the highest cans I know of at 100W is 4.2V), but this will work as he intended so if nothing else, I'll be better off than I am now.
Edited by sml1226 - 11/28/11 at 4:24pm
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post #10 of 11
11/28/11 at 7:00pm

You don't need to use such an elaborate design to use headphones out of a speaker amp. (i can't believe i'm calling 3 resistors elaborate) You can just use a single series resistor for each channel.

The three resistors are there to voltage limit into high impedance, and current limit into low impedance. It's taking the extra cautious approach.

If you just make sure you turn the volume down before plugging in the headphones, then are even remotely careful with the volume, you should be fine with a single series resistor.

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post #11 of 11
11/28/11 at 7:12pm
Thread Starter 
Quote:
Originally Posted by DingoSmuggler View Post

You don't need to use such an elaborate design to use headphones out of a speaker amp. (i can't believe i'm calling 3 resistors elaborate) You can just use a single series resistor for each channel.
The three resistors are there to voltage limit into high impedance, and current limit into low impedance. It's taking the extra cautious approach.
If you just make sure you turn the volume down before plugging in the headphones, then are even remotely careful with the volume, you should be fine with a single series resistor.

The goal wasn't actually even to use this though, it was to use something along these lines to achieve a specific output impedance, which I haven't quite figured out yet. And me being cautious at times, and blatantly stupid others feels the need to be able to do this without being overly cautious of the volume (though being able to get 5V RMS I guess it's not all that useful at that either)

Anyway, at this point I'm going to be trying different setups with this until I get the desired, whether that be measured or perceived, performance out of the amp.
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