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Old 11th March 2013, 11:50 PM   #2231
rjm is offline rjm  Japan
Richard Murdey
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"VA" (voltage X current) is the rated output, unrectified. It is normally stated for the transformer as a unit, rather than the individual windings. However, for the simple arrangements here you can assume 2x12 V and 120 VA means 60 VA per secondary winding, which is 12 V and 60/12 = 5 A, 10A with the windings in parallel.

What that means is: you can hook up the secondaries in parallel to a 1.2 ohm resistor and the transformer will comfortably run 10 A through it while maintaining the rated 12 VAC voltage. The resistor will dissipate 120 W.

You cannot, however, run the output through a bridge rectifier to ~18 V DC, and connect a 2.7 ohm resistor and a big filter cap and expect to pull 6.6 A DC (120 W equivalent). The transformer ratings are for AC circuits only.

The AC-DC conversion process involves considerable inefficiency because the transformer only conducts for a small amount of time during each AC cycle.

However, as has been pointed out at length in recent posts to this thread, we are not building a power amplifier and pretty much any power transformer is going to be "sufficient" to power the <100 mA required by these op amp circuits.

It's simply a matter of how much headroom you feel comfortable building in to your power supply.

Besides the "it sounds better" argument, my original logic for using high current power transformers was a) they don't cost that much more to build and b) external power supplies can be re-used to power other circuits like headphone amplifiers. The bigger they are, the more versatile they become.
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Old 12th March 2013, 12:08 AM   #2232
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While I'm in "justification mode", a word on op amps:

The Phonoclone 3 uses OP27 (OPA27), the datasheet lists "professional audio equipment" among the applications, which means it meets the criteria for low distortion. It's a bipolar circuit with low noise and high precision the main selling points. We are interested in the former, that latter is not very important since the circuits are AC coupled. The OP27 in particular has a low (8Mhz, same as NE5534) bandwidth so is more stable and has a fast settling time. It was recommended to me as being the chip used in the original. I can't confirm that, but it is certainly a good fit given we know about 47 Labs design.

The LT1028 and LT1115 are both similar to the OP37/OPA37, but with even lower voltage noise. ~1 nV/sqrtHz. All three are "for audio", particularly microphone and phono preamps, and have 40 Mhz or higher gain-bandwidth products. Between the LT1028 and LT1115 there is little to choose, the LT1115 seems to be more audio-specific in that distortion figures are given specific mention. The basic specifications seems quite similar however.

The OPA37 (and the OPA637) did not work well in the Phonoclone 3 boards, because the boards were not designed to handle the high bandwidths. The ICs from LT will run into the same difficulties. You'll have to do what vulejov did and a) put a cap over the first stage feedback resistor and b) bypass all the opamp power pins with small caps (preferably 0.1uF ceramic). That should be enough to keep everything stable. The bypass cap positions are on the latest Phonoclone 3 boards if you want to add them, but the capacitors are not included in the kit.
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Old 12th March 2013, 12:30 AM   #2233
Bibio is offline Bibio  Scotland
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thanks for clearing that up Richard.

no complaints this end as i think the kits are worth every penny. being my first kit it was easy with nice instructions. i had a look at the BOM on mouser and for an individual to buy just the parts was not far off the price of your whole kit here in the uk.

it's fun and addictive :-) i want MORE and can't wait till its all come together.
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Old 12th March 2013, 10:47 AM   #2234
vulejov is offline vulejov  Serbia
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Quote:
Originally Posted by rjm View Post
You'll have to do what vulejov did and a) put a cap over the first stage feedback resistor and b) bypass all the opamp power pins with small caps (preferably 0.1uF ceramic). That should be enough to keep everything stable..
Capacitor on first stage feedback resistor should be 100-220pF.. depends of the gain of the first stage..

I didn't bypass electrolyt cap.. in my board C4-C5 are 500uF and C6-C7 100uF electrolyt, C8-C11 only 1uF polyester without electrolytic..
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Old 12th March 2013, 10:57 AM   #2235
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Thanks vulejov. I meant that you used 1uF polyester instead of electrolytic at the positions closest to the opamps.
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Old 12th March 2013, 11:36 AM   #2236
vulejov is offline vulejov  Serbia
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That's it..
Looks like it's quite enough.. no need to add parallel a small ceramic..
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Old 12th March 2013, 11:41 PM   #2237
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What resistance did you choose for R9-R12?
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Old 13th March 2013, 05:25 AM   #2238
vulejov is offline vulejov  Serbia
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10R.. but I think that value should be smaller.. in first version there is no resistor and it works fine..
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Old 13th March 2013, 09:28 AM   #2239
simonov is offline simonov  Bulgaria
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OP27 is still among the best op-amps available. For those who wish to upgrade to something better I recommend OPA211 or OPA2211. Both having superior performance. However those are available in SMD packages only.

Anyways one should be aware then using a high speed op-amp in a non high speed PCB almost every time causes troubles. I always try to use a high speed techniques when routing my PCB's. Dual layer topology is a must.

Regards, Venci.
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Old 13th March 2013, 12:35 PM   #2240
vulejov is offline vulejov  Serbia
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I agree, but 100p-220p cap over feedback resistor is enough for troubleless work..
With LT1115 DC offset is 3-5 mV on first, and 20-50 mV on second chip.. but it has several times less noise and less distortion..
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