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PostPosted: 14 Jul 2010, 21:26 
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Hi, I too have used and like the TO252 version of the LR8N. But not everyone is good with SMDs so I usually recommend the TO92 version.

It is difficult to diy a power amp with a good preamp on board. Not impossible though. The best approach for me is to compartmentalize the stages. Metal shields (barriers) between the sections helps a lot. Careful layout is also key.

The advantage of separate mono blocks and a line stage preamp is flexibility. My amps can be placed near the speakers and the preamp anywhere within reason that you can get at it easily. There are some limits. Depending on the output impedance of the preamp (mine are about 2K) the cable length to the amps may influence the sound. Particularly by reducing the high frequencies. I have found with the OddBlocks up to 6 feet of nearly any good cable is fine. That way I can use fairly short (2 meter) cables to the speakers. The preamp is my own design (project is scheduled for the web - called the ForeWatt) and sits between the amps on a shelf.

I checked on the 5687 tubes for use as a SRPP driver. They will work fine, but have rather low gain. To get full output from a pair of KT88s you will need nearly 3 volts of input. This is well within the range for most line stage preamps. Two good sets of values for the 5687 are 180 B+, both cathode resistors 330 ohms, bypass cap on the lower one of 100uf. Gain is 13, current 10ma, Zin=150K, Zout= 460 ohms. At 220 B+ both resistors are 470 ohms and the only significant difference is the Zout is 400 ohms. Since both can do symmetrical +/-40 volt swings, I would opt for the lower B+. You would not need the heater bias complication. If you want to use NFB, and I do suggest you do a minimal amount in the Oddblocks. I would swap the resistor in the lower cathode to a 240 ohm (bypassed with the 100uf) and a 82 ohm to the ground. The NFB would go to the junction and the typical values shown in the project ought to work fine. For the 220 volt version I would swap the 470 ohm resistor with a 360 and a 120. The ratio should be about 3 to 1.

Good listening
Bruce

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PostPosted: 14 Jul 2010, 22:02 
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Thanks. I ended up with 200 volts and 470 ohm resistors and the results are excellent except for the noise, and I did bias the heaters at 1/3 B+. It sounds great, but now I'm also having a problem with rectification of local radio stations.

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PostPosted: 14 Jul 2010, 22:59 
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To completely change the subject but remain OT, I'm wondering if the LM317 could benefit from a capacitor on its output lead.

The datasheet indicates a cap on the output can improve transient response, and a 1uf tantalum is suggested.

That's using the IC as a voltage regulator, not a CCS.

However, the LR8 high voltage regulator IC calls for this same capacitor not only when used as a voltage regulator, but also when configured as a CCS.

If the LM317 and the LR8 operate on the same design principle that would seem to indicate that a 50 volt 1uf tantalum cap on the output of the LM317 to ground might improve its performance in the oddwatt. It's certainly not very expensive to do. I'd love to know if any tests with/without this cap have been done. It would be relatively easy to test an LM317 with and without the cap in CCS mode to see how it actually behaves. I may try unless someone already has.

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PostPosted: 15 Jul 2010, 09:43 
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sampleaccurate wrote:
To completely change the subject but remain OT, I'm wondering if the LM317 could benefit from a capacitor on its output lead.

The datasheet indicates a cap on the output can improve transient response, and a 1uf tantalum is suggested.

That's using the IC as a voltage regulator, not a CCS.

However, the LR8 high voltage regulator IC calls for this same capacitor not only when used as a voltage regulator, but also when configured as a CCS.

If the LM317 and the LR8 operate on the same design principle that would seem to indicate that a 50 volt 1uf tantalum cap on the output of the LM317 to ground might improve its performance in the oddwatt. It's certainly not very expensive to do. I'd love to know if any tests with/without this cap have been done. It would be relatively easy to test an LM317 with and without the cap in CCS mode to see how it actually behaves. I may try unless someone already has.

Google and ye shall find:

http://diyaudioprojects.com/Technical/V ... Regulator/

Why is there no 1uf cap on the output terminal of the LM317 in the CCS configuration as there us in the voltage regulator configuration? The circuits are identical except that the load takes the place of R2 in the CCS.

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PostPosted: 15 Jul 2010, 12:29 
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sampleaccurate wrote:
sampleaccurate wrote:
I'm wondering if the LM317 could benefit from a capacitor on its output lead.

The datasheet indicates a cap on the output can improve transient response, and a 1uf tantalum is suggested.

That's using the IC as a voltage regulator, not a CCS.

However, the LR8 high voltage regulator IC calls for this same capacitor not only when used as a voltage regulator, but also when configured as a CCS.

If the LM317 and the LR8 operate on the same design principle that would seem to indicate that a 50 volt 1uf tantalum cap on the output of the LM317 to ground might improve its performance in the oddwatt. It's certainly not very expensive to do. I'd love to know if any tests with/without this cap have been done. It would be relatively easy to test an LM317 with and without the cap in CCS mode to see how it actually behaves. I may try unless someone already has.

Google and ye shall find:

http://diyaudioprojects.com/Technical/V ... Regulator/

Why is there no 1uf cap on the output terminal of the LM317 in the CCS configuration as there us in the voltage regulator configuration? The circuits are identical except that the load takes the place of R2 in the CCS.

Excellent question. On the LM317 datasheet they show a 0.1uF on the input side of the LM317 "1A Current Regulator" example. It will be interesting to know if anyone has tried capacitors and if they improve performance.

There may be an answer in AN-181: http://www.national.com/an/AN/AN-181.pdf I'll have to take a closer look. Here is a small excerpt from AN-181.
Quote:
Two terminal current regulators can be made with fixed-output regulators; however, their high output voltage and high quiescent current limit their accuracy. With the LM117 as shown in Figure 8, a high performance current source useful from 10 mA to 1.5A can be made. Current regulation is typically 0.01%/V even at low currents since the quiescent current does not cause an error. Minimum operating voltage is less than 4V, so it is also useful as an in-line adjustable current limiter for protection of other circuitry.

Also, see this thread for CCS - Constant Current Source - Test Results and Comparison
Cheers

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PostPosted: 15 Jul 2010, 12:59 
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Here are the key words from the datasheet:

"Although the LM117 is stable with no output capacitors, like
any feedback circuit, certain values of external capacitance
can cause excessive ringing. This occurs with values between
500 pF and 5000 pF. A 1 μF solid tantalum (or 25 μF
aluminum electrolytic) on the output swamps this effect and
insures stability
."

"The 2 capacitors on the adjustment and output terminals are
optional for improved performance
."

There is no distinction made between operating the LM317 as a voltage or current source when this output cap is mentioned. They also suggest a cap on the adjustment terminal.

I didn't address the input cap, but the datasheet also calls for a 0.1uf input cap on the LM317 if it's more than a few inches from the last filtering cap.

Keaster: An ohm meter on the primary should tell you if the transformer is open. Check also to make sure there's continuity in the secondary and isolation between the primary and secondary IMHO.

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PostPosted: 16 Jul 2010, 09:18 
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sampleaccurate wrote:
Why is there no 1uf cap on the output terminal of the LM317 in the CCS configuration as there us in the voltage regulator configuration?

Hi.

I would concern more about the I/P cap as per LM317 data sheet..

As I already pointed out, a sand chip in the signal path can affect tbe sound regardless the chip there being a CCS or whatever DC current stabilizing function.

I would feel more comfortable to see the OddWatt design to include an I/P cap as suggested in the LM317 data sheet. This I/P cap will act as music signal bypass being more needed than the bypass cap for the cathode bias resistor of an O/P power tube.

c-J

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PostPosted: 16 Jul 2010, 09:56 
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Gio wrote:

Hi. Gio.

What part number of the transistors you used in the #10 & #11 columns of the comparison chart, which were rated "EXCELLENT" ?

c-J

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PostPosted: 16 Jul 2010, 10:49 
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sampleaccurate wrote:
Here are the key words from the datasheet:

"Although the LM117 is stable with no output capacitors, like
any feedback circuit, certain values of external capacitance
can cause excessive ringing. This occurs with values between
500 pF and 5000 pF. A 1 μF solid tantalum (or 25 μF
aluminum electrolytic) on the output swamps this effect and
insures stability
."

"The 2 capacitors on the adjustment and output terminals are
optional for improved performance
."

There is no distinction made between operating the LM317 as a voltage or current source when this output cap is mentioned. They also suggest a cap on the adjustment terminal.

I didn't address the input cap, but the datasheet also calls for a 0.1uf input cap on the LM317 if it's more than a few inches from the last filtering cap.

Keaster: An ohm meter on the primary should tell you if the transformer is open. Check also to make sure there's continuity in the secondary and isolation between the primary and secondary IMHO.

I built a remote PS once which feed power down a 1 meter figure eight power cord. The reg. got hotter than a Mexican Chilli Dog in July. I put a 0.1uf cap at the output of the PS and it ran cool. The reg. was ringing like a church bell on the first Sunday of lent.

After the inclusion of the 0.1uf cap the reg. ran cooler than a polar bear's nuts in the snow.

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PostPosted: 16 Jul 2010, 15:00 
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cheap-Jack wrote:
I would concern more about the I/P cap as per LM317 data sheet..

I would feel more comfortable to see the OddWatt design to include an I/P cap as suggested in the LM317 data sheet. This I/P cap will act as music signal bypass being more needed than the bypass cap for the cathode bias resistor of an O/P power tube.

If you bypassed the signal around the CCS from the input to ground you'd be grounding the drive signal for the lower power tube, and the SIPP wouldn't work. The CCS represents a high impedance to all audio frequencies. It draws the required bias current while maintaning a very high impedance from its input terminal to ground. I don't know if a 0.1uf cap would make any difference in the impedance at high audio frequencies, probably not, but I don't believe it's intended to bypass audio signal, it appears to be intended to bypass ultra-sonic frequencies that are above the audio spectrum. That's my take on it anyway when used in a CCS.

I think you're probably right about it being there, but I don't think it should bypass the signal to ground the same way a cathode capacitor does. It's way too small to do that anyway, and a cap that was big enough would render the amp inoperable for reasons stated above.

You're also right (I think) about how critical this part (the CCS) is to the performance of the amp. If the CCS sucks the amp will suck. However, Bruce has tested the LM317 extensively and from all of his experience the LM317 WITHOUT the caps is not the weak link in the amp. The specs (and sound) are great, and I'd have to agree.

Will putting the caps, or some of them, in the circuit make me feel better? Probably. Will it be audible? Probably not.

This topic DID provoke me to design some cool little CCS boards and high voltage regulator boards (PCB layouts) that will greatly simplify and neaten up the CCS and voltage sources in my amps. I'll make a bunch of them so I'll have them on hand when inspiration strikes.

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