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Old 9th September 2013, 12:54 AM   #1
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Default grado ra1 clone

Where i can find grado ra1 headphone amp pcb layout
I have allready made cmoy and now i am willing to make this little amp for my grado sr80i
greetings from Serbia !
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Old 11th September 2013, 11:16 PM   #2
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Join Date: Feb 2012
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Hello,

I just made one for my 80i/MS1's. Here are the diagrams i used, i first made one on a prototype board before i found kits for sale on ebay. Have a search for RA1 kits as i won an auction for ú6 which was a complete bargain as the board is very nicely made. I didn't use any of resistors/caps/vol pot supplied as i had better quality items to hand, but it was worth every penny just for the PCB. Well worth building one as it sounds very nice

Have fun...
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Old 13th September 2013, 06:08 PM   #3
prezden is offline prezden  Canada
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I just ordered a kit from Along 1986
DIY RA1 Headphone Amplifier Kit Power Amp JRC4556AD C39 | eBay
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Old 13th September 2013, 10:22 PM   #4
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Hmm. I always thought the RA-1 were using an inverting topology. (But no, seems it's not.) Otherwise you wouldn't need these high resistor values for the feedback network which are needlessly increasing noise and distortion here.

The 4556 ain't a low-noise king but it is better than a 10k resistor in terms of voltage noise, and it would not be bothered at all by driving feedback resistors downsized to 1k2 + 4k7. Not to mention that an impedance imbalance of 80-100 kOhms (+ input: 0-20k, - input: ~100k) is inviting input impedance distortion.

A 10k-20k volume pot would also be preferred for noise reasons. Sources that cannot drive 10k with authority seem to be about as common as hen's teeth nowadays.

If you feel like going inverting for minimum distortion, you might try a 20k pot and 22k and 100k resistors, or a 50k with 47k and 220k, or a 10k with 10k and 47k (assuming your source is not bothered by 5 kOhms of load when the amp is cranked all the way up).
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Old 14th September 2013, 03:58 AM   #5
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I barely understand half of what you wrote there but i will try and digest it the best i can

Thanks for the input..
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Old 14th September 2013, 08:21 PM   #6
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Basically, swap the 121k and 464k resistors for 1.2k and 4.7k jobs (regular 1% MF will do; you can go for 1.21k and 4.64k 0.1% if you really want, but that sort of precision is entirely unnecessary), and swap the 100k pot for something closer to 10k. Et voilÓ, a circuit that does exactly the same as before, except with less hiss and distortion. That's about as close to a free lunch as it gets.

I don't have good enough data on the 4556 to determine the improvement in distortion (going by the rather low input bias current for a bipolar, I'm guessing it may be less susceptible to input impedance distortion than other common opamps), but noise should go down by about 12 dB.
Unmodified noise levels are expected to be at about 27 ÁVrms across the audio bandwidth or -91 dBV, and would be audible on sensitive headphones and in-ears from about 120 dB SPL / 1 V up (even my old AKG K26Ps as small on-ear cans were rated 125 dB/V, though values that high are mostly reached by BA in-ears which can hit 130-135 dB/V).

Using the 4556 in inverting configuration may be most interesting when using it as a buffer and driving relatively high-impedance headphones, conditions that keep output stage distortion at bay while maximizing common-mode distortion in the non-inverting circuit (which is neatly eliminated when going inverting).
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Old 15th September 2013, 01:50 AM   #7
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Think i'm getting my head around this, the concept that resistors create noise is new to me, never knew that.

Found this handy site i've been playing with Electronics Calculators and got the following numbers,

stock
121k/464k = 4.83 gain w/ 72.54 ÁV, -83 dB relative to 1 Vrms noise

modded
1.2k/4.6k = 4.83 gain w/ 28.5 ÁV, -91 dB relative to 1 Vrms noise

Question is how does the value of the pot tie into this, for example if i change to 1.2k/4.6k and leave the 100k pot what happens?

Thanks again for the help
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Old 15th September 2013, 05:34 PM   #8
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I'm afraid that noise calculator doesn't work properly. For example, when using R4 = 1k and a noiseless opamp, there should be precious little difference between R3 = 100k, 1meg or 10meg, since the inverting input sees R3 and R4 in parallel (which is ~=R4) and voltage gain is 1+(R4/R3) ~= 1 in all of these cases. Yet the results returned are markedly different.

I've had a look at the code now. He's calculating voltage noise for all the resistors and the pot individually, applies gain and then RMS sums it all up. That's totally wrong. If you ever get anything resembling a correct result out of it, well, even a broken clock is right twice a day...
Quote:
Originally Posted by mcandmar View Post
Question is how does the value of the pot tie into this, for example if i change to 1.2k/4.6k and leave the 100k pot what happens?
Look at the impedance that the noninverting input sees. It is (pot wiper to ground resistance) in parallel to (top to pot wiper resistance + source output impedance). Worst case, wiper at half pot + source (-6 dB setting), where it becomes 1/4 of the whole shebang. So the noninverting input may see up to 25 kOhms with a 100k pot when driven from a low-impedance source. Kinda high if the opamp contributes as much voltage noise as a 5 kOhm resistor or so.
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Old 16th September 2013, 06:56 PM   #9
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PS: Here's an online opamp noise calculator that actually works, brought to you by my shoddy JS coding skills and the Web 1.0. (Look Ma, I found Math.round()! ) It's still a bit unfinished right now, but it does already work for calculating minimum noise level, and correctly as far as I can tell...

Resistors as dissipative elements have a certain amount of available thermal noise power all the time (a fundamental thermodynamics thing). Their resistance determines how much voltage noise (when unloaded) or current noise (when short-circuited) you get as a result.

Apart from a "noise" perspective, a "signal" perspective can be equally handy. You can say that using a lower-value volume pot, for example, decreases voltage noise. But at the same time, you can also argue that noise power has remained constant while signal power has increased (as the lower-value pot is drawing more current from the source). Both descriptions are equivalent, but the latter is better suited for understanding why lower resistor values tend to reduce noise (at least when working in the voltage domain, as often the case in audio).
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Old 17th September 2013, 01:59 AM   #10
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Nice work on the calculator, i need to order a few elecy bits so i'll get a few 1.2k and 4.6k resistors to experiment with. Does the size of the resistors make any difference to noise, e.g 1/4w, 1/8w etc?

Also whats your preference on decoupling caps around IC's/Opamps, do you favor mlcc ceramics or metal film?

I recently picked up an old analog scope so will be interesting to see if i can measure/quantify any difference. ...well interesting to me anyway

Thanks again..
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