High gain opamp based preamp for low output MC cartridges.
- Thread starter wyn palmer
- Start date
wyn palmer
Addicted Member
Once the supply decoupling board is designed and the parts identified I will certainly investigate it. In the mean time I can complete the current listing of parts with the remaining Mouser part nos. just to simplify the process.
wyn palmer
Addicted Member
The layout is nearly complete. Connectors have been added to permit easy insertion/removal of the load.
The MC load R/C can be inserted into the spring connectors and/or can be soldered into the board- (or of course can be soldered across the phono input plug, or wired onto a switch if wanted).
The spring connectors can be replaced with screw connectors if desired. They are cheaper and some might feel that they provide a superior connection.
The MC load R/C can be inserted into the spring connectors and/or can be soldered into the board- (or of course can be soldered across the phono input plug, or wired onto a switch if wanted).
The spring connectors can be replaced with screw connectors if desired. They are cheaper and some might feel that they provide a superior connection.
Hi Wyn. Don't think i've posted here before . However I did buy some boards from Sachin and built one of Hypnotoads 797 pre's. Ive built a few different designs over the years (I work in electronic test & dev -formerly pro audio for 20 years sadly no more) but that design was nice and simple and had cheap available pcbs. I stopped using it as it sounded a bit off in one channel - i found an error on the pcb when i dug it out the other day - the 3300pf (your suggestion i think) cap went to a different pin on left/right channels. Anyway after correcting that it now performs quite well 
Anyway I would really like to build your enhanced version. I recently bought a Benchmark AHB2/LA4 for use with my Martin Logan Ethos - and it has proved remarkably revealing so I'm keen to build a new phono stage
I would just like to build the phono section (I have several +_15 psu's) and set the buffer to unity gain to feed my la4 . Balanced out would be even sweeter !
Keep up the good work and thanks for sharing this .
Ben
Anyway I would really like to build your enhanced version. I recently bought a Benchmark AHB2/LA4 for use with my Martin Logan Ethos - and it has proved remarkably revealing so I'm keen to build a new phono stage I would just like to build the phono section (I have several +_15 psu's) and set the buffer to unity gain to feed my la4 . Balanced out would be even sweeter !
Keep up the good work and thanks for sharing this .
Ben
wyn palmer
Addicted Member
I use the preamp with an RME ADI-2 PRO FS acting as a preamp (ADC/DAC) a Benchmark AHB2 and a pair of ML Montis. The phono preamp gain is set to provide c. 350mv rms for a 0.25mv- 5cm, 1kHz input. This provides about 1.4v rms for an output at +12dB (20cm/sec) which is the point where most MC cartridges are just about to miss-track. The Benchmark, in high gain mode, requires 2v RMS for full power output, which can be provided by the single ended output buffer which can deliver 2.8v rms at +12dB, and I have tried it that way, with success.Hi Wyn. Don't think i've posted here before . However I did buy some boards from Sachin and built one of Hypnotoads 797 pre's. Ive built a few different designs over the years (I work in electronic test & dev -formerly pro audio for 20 years sadly no more) but that design was nice and simple and had cheap available pcbs. I stopped using it as it sounded a bit off in one channel - i found an error on the pcb when i dug it out the other day - the 3300pf (your suggestion i think) cap went to a different pin on left/right channels. Anyway after correcting that it now performs quite well
I would just like to build the phono section (I have several +_15 psu's) and set the buffer to unity gain to feed my la4 . Balanced out would be even sweeter !
Keep up the good work and thanks for sharing this .
Ben
Normally the RME is set for 0-6dB gain for the analog input, and I use the digital controls for volume control/PEQ/Loudness/balance/rumble etc. etc., although I mostly use the RME with all the frequency response profiles off.
There is currently no provision for balanced output from the phono stage per se. I can't say that I believe it matters. I've measured/listened to the results operated both ways (I added a SE- balanced stage) and there's no significant difference that I can tell. The RME and Benchmark work just fine with the usual pin strapping on the XLR connectors to permit single ended operation.
The preamp is extremely quiet- no mains hum is audible at anything like normal listening conditions. The SMPS produces no measurable hum harmonics. The hum that I get is due to pickup on the input cables- which would be benefitted by a fully balanced input- which is not going to happen, but even without that the hum levels are down nearly 90dB from full scale (+2.2dBv) on the converter.
If you do not use the buffer sections the fact that the second opamp has a DC servo around it, has capacitance load stability provisions, and has no series coupling cap means that it can handle a 2kohm/1000pF capacitive load without any problems, so no explicit buffer will be needed to drive a single ended line input, or inputs. If you're into hacking the board a bit, the input buffer amp can be used to provide the positive balanced output and the output buffer amp can be wired as a -1 stage directly to the phono stage output and used to drive the negative half of a balanced output connector. I've not tried it myself, but it should work. I can hack a schematic together for that, but it will require a small amount of extra work on the hoobyist's part. I don't care. It's all fun to me...
wyn palmer
Addicted Member
Oh, and if you don't want to mess about- the buffer amp that drives the record output is unity gain, but single ended, so you could just use that output and short the x2 buffer input terminal which would come from the volume control pot, to ground. It's still DC coupled with an offset of 1mv or less.
wyn palmer
Addicted Member
In fact, the buffer section can be provisioned to be a balanced output provided a volume control is not required. There would be a change in a resistor value- from 330 ohms to 47ohms, and we would have to add four jumpers/channel which would have to be hard wired as either a short or open in pairs.Hi Wyn. Don't think i've posted here before . However I did buy some boards from Sachin and built one of Hypnotoads 797 pre's. Ive built a few different designs over the years (I work in electronic test & dev -formerly pro audio for 20 years sadly no more) but that design was nice and simple and had cheap available pcbs. I stopped using it as it sounded a bit off in one channel - i found an error on the pcb when i dug it out the other day - the 3300pf (your suggestion i think) cap went to a different pin on left/right channels. Anyway after correcting that it now performs quite well
I would just like to build the phono section (I have several +_15 psu's) and set the buffer to unity gain to feed my la4 . Balanced out would be even sweeter !
Keep up the good work and thanks for sharing this .
Ben
The balanced output would have a 47 ohm source impedance/side, and that is to provide protection from instability caused by load capacitance.
I'll check with @HalfApt to see how easy it would be to change the layout, but I've already simulated the changed schematic and it would work quite well.
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keilau
AK Member
Yes, this is an expensive chassis because it is big and the shipping to go with it. It may just work for me if I decide to combine all the preamp function into a single chassis.
It has pre-drilled 4-in, one-out for RCA.
wyn palmer
Addicted Member
I use the solitary RCA socket pair as the phono in, and the other pre-drilled ones as line in, tape in, record out (buffered, no volume control), and line out (extra 2x gain, post volume control). All the RCA connectors are isolated from the chassis. All of the connectors, with the exception of the phono in, have their grounds hard wired together, and to the supply ground/signal ground on the output side of the preamp boards.
The phono-in ground (black wire + cable shield) is on the input side of the board and is connected to the output ground only through the board traces.
The case is always isolated from signal ground and connected to the ground wire of the IEC connector. The turntable ground goes to a binding post which is connected to the case close to the phono inputs for convenience- and wiring it to the TT ground does make a little difference, although there's very little hum/pickup even without that connection.
Having a separate enclosed section for the supply is very nice. I removed anodization at various points so the panels are electrically connected together.
The twisted together supply lines + ground from the SMPS go to the supply filter board (RCLC) and the filtered output lines are also twisted together and passed through a slot in the metal wall into the preamp area. It's quite effective. The LED/resistor is powered from the SMPS lines directly and is glued into the front panel hole. The Panasonic 10mH inductor I initially used is the one used in the AD797MCPRE recommended linear supply.
I no longer use that, but instead use a high self resonance 1mH choke from Bourns which is much cheaper, has lower DC resistance, higher rejection at 100kHz and above, and is not in an "end of life" situation.
wyn palmer
Addicted Member
We've added the ability to support a balanced output, but it comes at the expense of a recording output per se. and a volume control, and now the gain is permanently 69dB- so 250uv will produce 0.7v RMS, and +12dB will produce 2.8v RMS. This reduces the overload margin to 22dB from 28dB, but that should still be OK as it's likely that whatever preamp you drive it into will have inferior margins, and by most standards that's still pretty decent as it's at all frequencies below about 1MHz. The board uses on board hard wired jumpers to set the normal or the reduced function balanced mode output, but it's a simple job- akin to soldering in a couple of resistors.Hi Wyn. Don't think i've posted here before . However I did buy some boards from Sachin and built one of Hypnotoads 797 pre's. Ive built a few different designs over the years (I work in electronic test & dev -formerly pro audio for 20 years sadly no more) but that design was nice and simple and had cheap available pcbs. I stopped using it as it sounded a bit off in one channel - i found an error on the pcb when i dug it out the other day - the 3300pf (your suggestion i think) cap went to a different pin on left/right channels. Anyway after correcting that it now performs quite well
I would just like to build the phono section (I have several +_15 psu's) and set the buffer to unity gain to feed my la4 . Balanced out would be even sweeter !
Keep up the good work and thanks for sharing this .
Ben
The design is now frozen, so I will update the parts list with the options, making a few more changes in the process, and get the boards finalized.
Hi Wyn
Brilliant work thanks . When I build it, it will feed into one of the Benchmark LA4 Balanced in's . The line stage in it is quite literally another "benchmark" - state of the art specs. Its about the only line stage that has SNR/.THD as good as the AHB2 (its why they designed it)
Thanks again
PS I wish I had a pair of Montis ! I have the Ethos which were model below at the time . Do you just use the one AHB2 ? I had considered a second but with active bass in the Ethos it seemed overkill...
wyn palmer
Addicted Member
Yes, just one AHB2. The Montis are 4ohms (yeah, right!) which matches the AHB2 into 4 ohms, and the panel crosses over at c.300Hz with the bass unit being driven from it's own class D amp with a SMPS- in fact I pick up residual switching noise at a couple of frequencies, including c. 15kHz, from the electrostats unless I have a ferrite on their power cord. The AHB2s are dependent on the performance of modern opamps for their excellence. When the Quad 405 was born they used crappy LM301s and couldn't pull it off with essentially an identical architecture to the AHB2. Ah, the joys of modern semiconductors...
The RME is equivalent in performance to the Benchmark DAC3, but with an even better ADC at the front end, so I just immediately go to digital these days and convert back to drive the AHB2- hence the SOTA phono preamp design.
The Montis are rated at 200W into 4 ohms if I remember correctly, and I don't want to overdrive the panels. I used to wreck sections of a pair of ESL63s on a regular basis...
Every now and again, such as when I'm playing tracks from the 45RPM version of Janis Ian's "breaking silence", the clip indicators appear on the AHB2, but that is extremely rare, the music is very loud, and the "clipping" is completely inaudible...
I also use a subwoofer and the XTZ room analyzer pro system to set up the bass etc. and the performance is quite good. The subwoofer also uses a class D amp.
It's a bit curious, but there are times that I have an ICEpower class D, a Pudseys Hypex class D and a BASH class D, and my phono stage all operating with SMPS and the only "analog" bits being the phono/buffer stage. Even when the AHB2 is in the supply is still class D.
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Nice. The Montis are just a slightly bigger panel than mine - same B&O icepower amp in the bass cabs same power etc. Same dip at 20K
They crossover at 375Hz as opposed to your Montis 340Hz due to the smaller panel in the Ethos. Not managed to get my clips lights on yet - but I am in a medium sized UK home . And have neighbours Then again only had the amp a few weeks... I was using a pair of Hypex NC500 modules before the AHB2 - plenty of wallop but i was never entirely with treble quality, it had something intangibly wrong with it i thought - playing music i've been familiar with for years it seemed a little off at HF . Reverberation is much better produced through the AHB2 and generally it sounds more tonally correct i feel. Anyway it all sounds great and I'm very excited to build a nice high performance MC stage again . Your work on this is very much appreciatedPS Switching supplies were our enemy back when I worked in audio - they were just not up to the job. We used them for digital circuits at AMS Neve, but never at Amek (another long gone UK console company) . Properly designed ones are great now . No hum
wyn palmer
Addicted Member
I have Ncore400s. Somehow the HF seems- muted- into the Montis, although they don't seem to be noticeably different when I measure the room with them.Nice. The Montis are just a slightly bigger panel than mine - same B&O icepower amp in the bass cabs same power etc. Same dip at 20K
PS Switching supplies were our enemy back when I worked in audio - they were just not up to the job. We used them for digital circuits at AMS Neve, but never at Amek (another long gone UK console company) . Properly designed ones are great now . No hum
The AHB2s and the Rogue M180s have a more similar sound, believe it or not. You have to be listening closely to hear the difference, and even then I've not done a proper ABX test to find if the impression is correct- the gain of the two amps is very different and it's hard to get the loudness right without screwing up the Rogues, especially since they behave very badly with high source impedances.
Hopefully you will like the phono stage. My main other one is a SUT plus Hovland Hp100 and the tonal/presentation differences are surprising, and not always better, although on balance I prefer the SS one.
I don't think that you can buy a significantly quieter or with a more accurate RIAA or lower distortion one out there, so there's that at least.
wyn palmer
Addicted Member
My listening room is c. 44x17x24 and it's hard to fill with sound. It's not the best arrangement...Nice. The Montis are just a slightly bigger panel than mine - same B&O icepower amp in the bass cabs same power etc. Same dip at 20K
PS Switching supplies were our enemy back when I worked in audio - they were just not up to the job. We used them for digital circuits at AMS Neve, but never at Amek (another long gone UK console company) . Properly designed ones are great now . No hum
mine is about half that size ! muted treble on the nc400. yes the nc500 treble was kind of recessed but not measurably . the highest notes female singers hit seemed to limited somehow is what i thought subjectively when listening . i was pretty impressed with them on the whole though
wyn palmer
Addicted Member
I agree. The Rogues can change their sound by swapping out tubes. The tubes are not original equipment, save for the KT120s, they are NOS and were chosen because they sounded right to me, so it was quite a surprise that the Rogues and the AHB2 sounded so similar and the Ncore400 sounded so different.
As a confirmed objectivist it's a bit of a conundrum for me...
The Rogues also have a very inductive output impedance, which exacerbates the HF roll off of the electrostatics but it seemingly doesn't matter.
Anyway, enough of the commiserations, this is a thread about this new preamp/phono stage, and now it's time to solicit board orders as I plan to buy preamp and PS filter boards as soon as the layout is complete.
