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Upgrade Pro-Ject Head Box II headphone amp
Upgrade Pro-Ject Head Box II headphone amp
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Old 6th July 2020, 08:43 PM   #31
maurihal is offline maurihal  Italy
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Quote:
Originally Posted by maurihal View Post
it is better that you publish the schematic

it's me, not you, obviously, who have to publish the schematic
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Old 7th July 2020, 08:41 PM   #32
maurihal is offline maurihal  Italy
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Here is the schematic of the amplifier how it is working right now.
The choice of the final oversized tranasistors is not part of the 'recipe': they are the ones I managed to combine according to my idea (it is possible that it is not new, however).
In any case, in my opinion, the BD139-140 of current manufacture are not suitable for authoritatively driving headphones, and they give a flat sound.
Someone said a long time ago, in the seventies: "NPN, PNP transistors are as truly matched as men and women of the same height and weight".
So I matched them following another theory.
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Old 10th July 2020, 03:32 PM   #33
maurihal is offline maurihal  Italy
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I followed the suggestion of sgrossklass:
Quote:
Originally Posted by sgrossklass View Post
Honestly I'm not convinced that this setup (as shipped) even is more effective than some passive filtering. You could rip out the NPN, replace its B-E junction with 820 ohms and still add the protection diode (c), and I bet you'd be no worse off than stock.
I can't say for sure that it's better, but it's certainly not worse. It is certainly more robust.
The amplifier thus described sounds quite good. We will see if we can understand the nature of the problem that occurs in some passages of the female voices.
It must be remembered that there is a review on Head-Fi entitled "Pro-ject Headbox II: First impressions" which, unfortunately, is perfectly in agreement with my own impressions.
The final transistors are matched of HFE and VBE, but also of a third characteristic, which is my 'trick' (or my illusion).
I want to quote John Linsley Hood's 'Audio Electronics' book:
“In many ways the Shaw/Baxandall quasi-complementary output transistor circuit is preferable to the use of fully complementary output transistors, since, to quote J. Vereker of Naim Audio, ‘NPN and PNP power transistors are only really as equivalent as a man and a woman of the same weight and height’ – the problem being that the different distribution of the N- and P- doped layers leads to significant differences in the HF performance of the devices. Thus, although the circuit may have a good symmetry at low frequencies, it becomes progressively less symmetrical with increasing operating frequency.
With modern transistor types, having a higher current gain transition frequency (that frequency at which the current gain decreases to unity), the HF symmetry of fully complementary output stages is improved, but it is still less good than desired, so the relative frequency/phase characteristics of each half of the driver circuit may need to be adjusted to obtain optimum performance.”
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Old 17th August 2020, 09:25 PM   #34
maurihal is offline maurihal  Italy
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Default Pro-ject Head Box II - las mods (maybe)

Hi folks!
Perhaps the time has come to close the case: the final changes should be that of the attached diagram.
There are two important changes compared to the latest version.
I have definitively adopted the OPA1652 (18 Mhz) instead of the OPA1642 (11 Mhz).
In this configuration, for me, it is by far the best compared to many tested.
Here is the list:
JRC2068D (stock), JRC4562D, JRC5532D (good, better than NE5532), LM4562 (National), NE5562 (TI), OPA1612 (here it oscillates heavily), OPA1692, OPA1656, OPA1642, OPA2209.
The replacement of OPA1942 only occurred when I replaced the ouputs with the following 'married couple' (remember Vereker's phrase): MJE15028 (30 Mhz) + 2SB1185 (Rohm, 70 Mhz).
I paired them based on HFE (measured at 6ma, having only a Mega328), and the VBE, unfortunately, is adapted with a 5% deviation. This is probably the limit to be respected.
I think I understand (this is the trick I was talking about) that the most important thing is to synchronize the phase of the two half-waves of the signal, as happens in the famous Naim amplifiers with the adoption of two compensation networks. Here another method is used to achieve the same result.
Now the amplifier sounds 'good', balanced throughout the sound range, with deep basses. I can listen it for hours without fatigue.
Thanks again to Sgrossklass for the suggested modification (a resistor instead of a transistor), which proved to be perfectly effective.
Regards
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Last edited by maurihal; 17th August 2020 at 09:34 PM.
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