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Old 16th March 2013, 04:43 PM   #11
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Quote:
Originally Posted by Ken Newton View Post
DAC directly to my power amp. Wow, what an improvement! The sound is now SO MUCH better that, for now

Hafler has made generous use of quality JFETs, and simple yet smart circuit discrete topologies. The only places they appear to have taken the easy route is for the voltage regulators, and the headphone amplifier, both of which are IC implementations.
Ken,

I've experienced the same thing with a Benchmark DAC w variable output direct to a AES tube power amp. I used it this way for a long time until I needed analogue input in particular a phono. BTW, here is a photo of Hafler simple phono stage which sound quite nice to me.

The headphone section is quite pitiful and don't use it.

Lastly, the preamp has opened up quite a bit from its 4 year period of non-use. What would you recommend to improve the power supply?

Mike
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Old 16th March 2013, 05:13 PM   #12
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Hi, Mike,

I suppose, you could try applying the usual suspects of generic DIY power supply mods. You know, audiophile e-lytic caps., newer 3-terminal regulators., etc. However, as I indicated is my experience, sometimes (rather often, actually) it's simply less of a headache to replace the component for something which you positively know sounds better, to you. This can even be a less costly route than 'upgrading' your 915 with premium priced audiophile parts. Upon completion of an upgrade, you still may not have obtained the subjective improvement you had hoped to. True, you wouldn't learn as much, nor get the fun and sense of accomplishment that comes from modifying your own equipment, but you just may end up getting more enjoyment from actually listening to music.

Sorry, that I'm not able to guide you better on this.
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Old 17th March 2013, 04:38 PM   #13
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Quote:
Originally Posted by mjatabor View Post
Ken,

....The headphone section is quite pitiful and don't use it.

Lastly, the preamp has opened up quite a bit from its 4 year period of non-use. What would you recommend to improve the power supply?

Mike
Mike,

I took another look at the 915's schematic. I did notice one particular detail of the power supply implementation that we don't find often utilized in more recent products. Which is, that all amplification blocks appear to directly wired to the same +15V, -15V supply rails. I didn't readily find any provision even for passive decoupling of noise and inter-stage power supply disturbances, only local passive bypassing. It's pretty much standard practice today to find multiple independent supply regulators, or, at the least, passive decoupling being utilized to isolate supply rail induced disturbances between circuit blocks. I'm not sure there is much you do to change this without hacking up the PCB traces, possibly damaging the unit and rendering it worthless even to re-sell.

I can fully believe that the headphone section sounds none too pleasing. It is comprised of an single NE5532 op-amp per channel. It features a 150 ohm current limiting resistor, placed between the 5532 and the output jack. This effectively sets the output impedance at 150 ohms. This is too high for many audiophile headphones. Hafler might have utilized two or more NE5532s in parallel (many hands make for light work) to help deliver enough current to enable use of much lower value current limiting resistor, and increase total power dissipation capability. The additional cost and implementation trouble would have been minimal, I should think. On top of all that, the circuit appears to be connected directly to the same supply rails as every other gain block in the unit. Seems like incongruous cost based shortcutting, given the nice discrete JFET circuitry included elsewhere in the unit.
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Last edited by Ken Newton; 17th March 2013 at 04:58 PM.
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Old 24th March 2013, 05:43 PM   #14
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Quote:
Originally Posted by Ken Newton View Post
Mike,

I took another look at the 915's schematic. I did notice one particular detail of the power supply implementation that we don't find often utilized in more recent products. Which is, that all amplification blocks appear to directly wired to the same +15V, -15V supply rails. I didn't readily find any provision even for passive decoupling of noise and inter-stage power supply disturbances, only local passive bypassing. It's pretty much standard practice today to find multiple independent supply regulators, or, at the least, passive decoupling being utilized to isolate supply rail induced disturbances between circuit blocks. I'm not sure there is much you do to change this without hacking up the PCB traces, possibly damaging the unit and rendering it worthless even to re-sell.

I can fully believe that the headphone section sounds none too pleasing. It is comprised of an single NE5532 op-amp per channel. It features a 150 ohm current limiting resistor, placed between the 5532 and the output jack. This effectively sets the output impedance at 150 ohms. This is too high for many audiophile headphones. Hafler might have utilized two or more NE5532s in parallel (many hands make for light work) to help deliver enough current to enable use of much lower value current limiting resistor, and increase total power dissipation capability. The additional cost and implementation trouble would have been minimal, I should think. On top of all that, the circuit appears to be connected directly to the same supply rails as every other gain block in the unit. Seems like incongruous cost based shortcutting, given the nice discrete JFET circuitry included elsewhere in the unit.
I have an SE100 which I modified with CRC filtering on the stock rails to help decouple the channels from each other. The SE100 design had provisions that allowed rail resistors to easily be installed and the traces cut. I used 8.2 ohm 1 W resistors here. The SE100 was the precursor to the 915 and utilized a very similar line stage circuit, so Hafler might have implemented this feature on the later 915 as well. I looked at the pictures posted of the 915 and I do not see it there from the top, but its possible there might be something on the backside of the board.

I posted a picture of my modded SE100 with the rail CRC resistors installed, for those that have trouble spotting them. They are the 4 larger light blue ones in the center of the board. Two of them are sitting vertically and have their wires soldered together, the other two are laying down horizontally.

For the headphone output, I just pulled the NE5532, I don't use the headphone output and the NE5532 was just another draw on the little stock 6VA rated transformer that was not needed. I have everything on my work bench tied through a Kill-a-watt meter and the power draw dropped from 3 VA TO 1 VA after the NE5532 was removed. I know that the kill-a-watt is not exceptionally accurate at this low level, but I was a bit surprised that it registered that much of a power drop with the 5532 in the circuit.

Front_Upgrd_A.jpg
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Last edited by Chamberman; 24th March 2013 at 05:45 PM. Reason: Forgot to attach picture
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Old 25th March 2013, 12:26 AM   #15
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I compared your SE100 to my 915 and there are quite a few similarities. Looks like the 915 has a few more op amps.
Did you notice any sonic improvement by removing the headphone opamp?
The area that you added your resistors is quite similar w/ the 915 also including an additional opamp.
BTW, did you substitute the factory poly caps w/ Wima. That would be an easy change for me to do, but I'm not sure if it will offer any sonic benefits. What do you think?
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Old 25th March 2013, 02:07 AM   #16
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Join Date: Jun 2001
Location: USA, Grand Prairie, Tx
Quote:
Originally Posted by mjatabor View Post
I compared your SE100 to my 915 and there are quite a few similarities. Looks like the 915 has a few more op amps.
Did you notice any sonic improvement by removing the headphone opamp?
The area that you added your resistors is quite similar w/ the 915 also including an additional opamp.
BTW, did you substitute the factory poly caps w/ Wima. That would be an easy change for me to do, but I'm not sure if it will offer any sonic benefits. What do you think?
I did not check the before and after sound of the preamp with the NE5532 removed. It would be easy enough to do just remove it and try it out.

I did replace 6 of the caps in the linestage. They were originally evox polycarbonate's and I swapped them for Wima Polypro's. Yours in the 915 however are already the better quality polypro film caps and I cannot see you getting any benefit by swapping them out for Wima's.

Unfortunately most of the circuit is already pretty decent, so tweeking gets harder. You could try swapping out some of the critical resistors in the line stage for low noise Dale RN60's. Beyond that you will have to start looking at building up a different power supply and ditching the 7815/7915 regulators. Or depending on your skill level you could use your input stage buffer as a passive preamplifier. That is what I did with my SE100 (actually I added the buffer the 915 already has it) and I posted a thread on this today.
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