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Old 3rd October 2017, 08:51 AM   #1
Masca64 is offline Masca64
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Default preamp to build

hi people,
I've some BB OPA2134 in my laboratory, so I'd build a complete preampl hi end with op amp . It means:
- input stages (RIAA equal stage included) ;
- tone control stage ;
- buffer stage ;
Since two months I'm looking for a valid scheme for start the opera .
In DIY forums, I've read a lot of opinions from everyone...a lot of opinions, a lot of scholl of thougts...all more or less accettable. One thing I understand at the end: dont exist an absolute parameter/value; whatever technology -op amp-all fet-mosfet-bjt-mixed- and whatever circuit tipology. The best judge at the end of the opera will be our ear; it means : how play music the creature in my opinion.

That said: I've found this Nelson Pass preampli 2012 very interesting to build:
Preamplifier 2012 (1) - introduction and line-in/tone/volume board (110650) - Elektor LABS
I would know it's possible to swap NE5532 ics with OPA2134 ( for me yes ) .If someone has built it, let me know how it play an other any important considerations...
in add:
Then:I'll be happy to read more suggestions about projects built with other tipology, others components ( fet, mosfet, bjt .....), and how they sounds ...I've no limits to costs -for example; I'm purchasing 4 OPA637 just now to do some tests- ( it's a long time I no build pre and ampli , so I can spend some money for use the best ) .
p.s -after, I would like match an hi fi ampli with mosfet
Thanks in advanced to everyone reply
have a nice day
Masca
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Old 3rd October 2017, 09:30 AM   #2
Masca64 is offline Masca64
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errata corrige:
'That said: I've found this Nelson Pass preampli 2012.....'
read
'That said: I've found this Douglas Self preampli 201.....'

please.
Sorry for mistake
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Old 3rd October 2017, 04:18 PM   #3
tER299 is offline tER299  Netherlands
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A quick glance at the current noise of the OPA2134 seems to indicate that the current noise (for which the 5532 was chosen in the phono part of the preamp) is only slightly higher (3 vs 2.7). Using Samuel Groner's opamp measurements as a backdrop however, seems to suggest some increased THD when loading the OPA2134, as would be done in this preamp. This would probably not be audible though, given that I've never seen a cartridge measure better than 1% THD, but YMVV.

If you have free OPA's available, why not try. I intend on building the same phono preamp, but you should note that the 2SA1085 is out of production (I bought the last 4, of 8 needed, from Reichelt), so you might want to find a replacement. The tone control part has been improved upon somewhat, and this newer version can be sourced from Carl Huff.
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Old 4th October 2017, 08:19 AM   #4
Masca64 is offline Masca64
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good morning forum...thanks for your reply ter299...let me take a look to the pdf of Samuel Groner...thanks for the suggest about Carl Huff, I'll contact him to get others technical info about the project. Sorry but I don't understand your
' ...you should note that the 2SA1085 is out of production...'
Watching the scheme I can't see use of this bjt ....I'm wrong? .
Anyway...I'm continuing for the while to search others valid scheme that use OPA627 or analog device audio op amp (like AD843 or AD797 ...they seems sound clean , warm...very well too ) before starting build this project ; other suggest, comments..opinions are welcome .
Thanks again.....nice day ter299 and to everyone...
Masca
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Old 5th October 2017, 02:14 PM   #5
indianajo is offline indianajo  United States
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In the real world, power supply details and packaging details matter as much or more than the active element. After thrashing through a commercial kit that was awful, I learned the following:
1 Don't put your power transformer and AC switch near the high gain elements. Transformer in separate faraday cage works best.
2. double pi input filter on DC in helps kill hum.
3. chokes on your DC input from the AC box keeps RF interference out of your circuit
4. Separating analog ground from case ground helps hum. That includes the rings of the RCA jacks from mag phono cartridge. Shell of tone arm ground goes to case.
5. Decoupling inputs with ceramic caps to analog ground helps reduce RF interferance. I used 33 pf.
6. Driving cables to the power amp, if over 2 m long, you need more current than a 2000 ohm drive op amp will push. The drive stage can be low gain so a lower noise op amp can work. Peavey uses 4580. As can current buffer ICs work (I haven't tried).
Also on the high gain stage, op amps with more than 7 v/usec slew rate need local power supply decoupling and ceramic bypass capacitor around the feedback resistor to avoid oscillation. I used one .1 uf +rail to - rail within 2.5 cm of two RIAA inputs, that was good enough. 33 pf around the feedback resistors.
Have fun.
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Last edited by indianajo; 5th October 2017 at 02:19 PM.
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Old 6th October 2017, 08:27 AM   #6
Masca64 is offline Masca64
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Hi indianajo,
at the beginning thank you so much for your participation ,
I’ve read carefully your precious suggests; I’m tell you my opinion point by point:
1- you mean AC section (transformer –switch-fuse-ac filter) in a metallic box separate from preampl case? I just thought to be enough put a metal shield between ac section and rectifier circuit .
2- ok, a pi RC filter on positive power supply rail and one on negative power supply rail;
3- ok a choke before each RC filter. Value of it ? a VK200 could be good?
4- This always have had my dubts. Grounding? Shielding? I always thought a connession called ‘ star centre’: one only point to connect together all grounding from each pcb ; why separate? Ok for rings rca.
5- You mean connect the input cold pole to gnd of the preampli board by a capacitor 33pf?
6- There’s no problem , amplifier is so close to preampl.(max 50cm distant)
7- I think I’ve no need to insert a 4580 ic .
Well…yes surely slewrate of OPA overtake 7v/us ( I remember 20) ; yes of course: I put in practice your precious suggestions…

Thanks a lot indianajo… may be I’ll ask you further suggests ….
H.a.n.d. forum ( have a nice day )
mas
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Old 6th October 2017, 05:32 PM   #7
indianajo is offline indianajo  United States
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A steel bulkhead between the AC power supply section, and the amp section, divides the box into two faraday cages. The most common solution before 1980 but using a wall transformer for the AC/DC converter is more common now.
I just did LCRC on the +18 from the wall transformer going into the preamp box, splitting the 18 into +-8 in the box with two zeners resistor stack. LC wasn't good enough, hummed a little, even with filter caps after the zener regulator. L was a choke from AC feed to a PCAT switcher supply, equivalent to the chokes used in all televisions these days before the surge supressor, rectifier, , & cap. ~20 turns on a ferrite toroid. R was 1 ohm.
Just putting RCA jacks through holes in the metal case involves the case ground (safety ground) in your analog ground. Messes up the star with the case being a big loop. As above, the metal case is the tone arm shell and also the AC safety ground if a power transformer or AC switch is inside the amp case. I put rubber o rings under RCA jacks, helped hum a little, but not as much as LCRC filter above the final solution.
No analog ground input RF filtering, Hot was RF filtered. I mean connecting the input hot (center of RCA jack) to analog ground (ring) with a 33 pf cap. My RA88a was done on the PCB, not actually at the jacks. One thing the packaging engineer at Herald did right. Putting the 115/17CT transformer & AC switch right next to the gain 50 RIAA stage was his biggest mistake; hummed even with the expensive grounded case transformer used.
Close amp and short cable means 2000 ohms drive summing op amp should be enough drive. I mean the voltage swing table on the datasheet shows a 2000 ohm load. Many more modern op amps are 600 ohms.
My mixer was hissy hummy mess when I bought it, 50x 4558 op amps was the cause of the hiss. 33078 needed the bypass capacitors noted above, quieted hiss right down. Not the most premium of op amps but that mixer is now the least of my problems so I never installed the LM4562 I bought to replace 33078.
Best of luck on a silent speaker out when the inputs are off. Mine is quieter than the gas pilot on the room heater.
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Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 6th October 2017 at 05:41 PM.
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Old 6th October 2017, 07:42 PM   #8
tER299 is offline tER299  Netherlands
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Quote:
Originally Posted by Masca64 View Post
good morning forum...thanks for your reply ter299...let me take a look to the pdf of Samuel Groner...thanks for the suggest about Carl Huff, I'll contact him to get others technical info about the project. Sorry but I don't understand your
' ...you should note that the 2SA1085 is out of production...'
Watching the scheme I can't see use of this bjt ....I'm wrong? .
Anyway...I'm continuing for the while to search others valid scheme that use OPA627 or analog device audio op amp (like AD843 or AD797 ...they seems sound clean , warm...very well too ) before starting build this project ; other suggest, comments..opinions are welcome .
Thanks again.....nice day ter299 and to everyone...
Masca
Well, you talked about a RIAA preamp, and given that you pointed to the 2012 preamplifier, I assumed that you intended on using the phono preamp circuitry described in these articles. That circuit relies on the aforementioned 2SA1085.

I should note that the circuit seems to hold up pretty well under testing, given that it's pretty much on par with Bob Cordell latest project, and those two are the best performing phono preamps I know of.

I should note that, if you were to use NE5532's or LM4562's (first one used in the 2012, second one in the 2012 and exclusively in Self's latest), you don't really have to worry a lot about noise and ripple from the power supply itself. Both opamps have pretty great PSRR. While indianajo's advice is very helpful, I doubt Self's designs need additional decoupling. I always use toroidal transformers to reduce hum, but that advice is probably trivial.
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Old 16th October 2017, 07:42 AM   #9
zdr is offline zdr  Belgium
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Tone controls still in use in amps? If they are not a must, you can consider an awesome passive attenuator:

Arduino based LDR volume and source selection controller
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