Why does the APT Holman preamp sound so good?

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
i worked at APT from 1979 until they closed. the Holman phono preamp is a rather unique preamp. the phono stage is a discrete op amp with a JFET/BJT diff amp (the JFET is the + input, the BJT is the - input). the JFET is a device selected at initial test by testing for noise. about 1 out of 5 2N5484's passed this test, the remainder were used as integrator discharger switches in the APT-1 power amp.
the output stage of the phono preamp was a class AB, biased a bit "hotter"... more of a "class AAB" stage. the RIAA network used 1% resistors, and some of the values were determined at initial test, so the RIAA characteristic was accurate to better than 0.1%. the rail voltages for the phono stage were filtered with capacitance multipliers, and so tended to be very quiet.

with the addition of some stricter power rail bypassing, the TL072's can be replaced with TLE2072 op amps if you want to update to more modern op amps
 
"Is the Pass that bad?"

No

I have found 90% of people cannot hear in the audiophile sense, and of the 10% that can, 90% of those pick the worst unmusical sounding gear.

The 1% that could tell whether something was musical never seemed to have any money.

I would describe the APT as uncolored, but lacking in dynamics, lifeless.

"Any schematic ?"

Unfortunately, yes, but I am moving and not likely to find it anytime soon.


I completely agree. I loved that pre for what it could offer. But, sadly, when I could afford a newer pre...it got shelved.
 
Wao this post is from December 2004 but here I am

I own a APT Holman preamp and is true that is lifeless with poor dynamics but this is due to the use of cheap electrolytic capacitors and old TL072 op amp.
But replacing those capacitors where the signal passes with Nichicon or Black Gate capacitors and changing the TL072 with Burr Brown OP2132 it start to compete in the Audiophile territory.
This preamp have more features than any other preamp made, like dubbing, 2 tapes, ext processor, etc. and a enviable phono stage.
For those who own a APT Holman here I put a scan of schematic better than the ones available.
Change Ic1, Ic5, Ic6, Ic7, Ic8 if you are going to use Tone defeat but if you will like to use Tone controls change Ic 9, 10 and 11 also for OPA2132
Ic2,3 and Ic4 works with Tape input, if you are not going to use them can leave the TL072's
Change signal caps C24,C124,C31,C131,C33, C133,C40, C140,C43,C143,C52,C152,C68,C168,C69,C169 with Black gate or Nichicon caps
Change C12 and C112 0.47 uf with something good like Solen if you use Phono Stage and C13,C17,C113 and C117
In the power supply change the rail caps 1,000 at 35v even if you see them good and the 4 little 10 uF at 25v C60,61,63 and C64.

After that you will hear an audiophile preamp with the enviable functions that the Holman preamp gives.

Good luck, Cesar
 
Chickenwatt - NICE!

I have been dormant with my stereo hobby for a long time. Now being an empty-nester, I have rediscovered it.

I have an old Apt-Holman preamp that I have fired up after a long time. Yes it does sound a little lifeless but very smooth. I have been thinking of upgrading some internal components and came across this thread - an it is now fairly current!

About 22 years ago, I sent my Apt into GasWorks and they replaced some of the caps. Really made a big difference, especially on the high end. They took a highligher to indicate the replaced cap on the schematic on the bottom of the unit.

I'll take a look at your recommendations in the near future to upgrade the rest of the circuitry. My source selector, along with other switches need cleaning. And the Mute button sometimes needs to be fiddled with.

Thank you for the information!
 
Member
Joined 2008
Paid Member
Here you go, Andrew :).

Had to resize the original down to 80% due to the image height limit, because I did not want to slice it into 4 parts. Please somebody check whether everything's still legible.
 

Attachments

  • Apt_Holman_Schematic_01.png
    Apt_Holman_Schematic_01.png
    621.5 KB · Views: 955
  • Apt_Holman_Schematic_02.png
    Apt_Holman_Schematic_02.png
    436.1 KB · Views: 911
  • Apt_Holman_Schematic_notes.png
    Apt_Holman_Schematic_notes.png
    150.9 KB · Views: 872
Member
Joined 2008
Paid Member
The patent is still available via Google, attached it here.

The other links are not available via the wayback machine, so probably lost forever.
One more thing Google found was this: http://www.amplimos.it/images/apt-holman-preamplifier-service-manual.pdf
Another link, but the pdf is much too large with almost 8MB. I could slice it up and attach the single pages here, but 51 pages is a lot of work...
 

Attachments

  • US4117412.pdf
    803.5 KB · Views: 163
  • holman_patent.png
    holman_patent.png
    22.3 KB · Views: 804
Member
Joined 2011
Paid Member
I was curious to know whether an FFT of a 1kHz input, in LTSPICE simulation, might show anything revealing.

Apparently not, except maybe that THD @ 1kHz was a wee bit on the high side, and only odd harmonics were present. Even harmonics were tiny. But I picked BJT types at random, and heaven only knows how accurately the LTSPICE presupplied models of those BJTs, include device nonlinearities.

_
 

Attachments

  • holman.png
    holman.png
    83 KB · Views: 797
  • apt_fft_sim_schematic.png
    apt_fft_sim_schematic.png
    26.2 KB · Views: 327
  • apt_holman_preamp_1kc_FFT.png
    apt_holman_preamp_1kc_FFT.png
    12 KB · Views: 276
  • apt_thd.png
    apt_thd.png
    15.8 KB · Views: 207
Last edited:
Member
Joined 2008
Paid Member
You could try to add an ".opt method=gear" and set the timestep to 1u or even less for some more precision. numdgt=7 is just as good, as the help file states that the simulator uses 6 digits by default and 7 digits max. You'll have to remove most of the capacitors (if not all) to get rid of that 50-to-150-dB-slope for a more realistic representation of the spectrum. Shorting out the input coupling cap is easy, but you'll probably have to replace the others with voltage sources for a given DC operating point.

Did you try any of Bob Cordell's models?
 
Member
Joined 2011
Paid Member
Naah, no more effort on this from me; I got the data I wanted despite the imperfect setup. Other highly motivated persons might decide to dig deeper. The good news is that the WILDLY asymmetric long tailed pair, appears not to contribute 2nd order distortion; at least, not in this simple simulation.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.