Phono preamp of Rotel 1592

Discussion in 'Audio Hardware' started by Andrew Montreal, Aug 9, 2022.

  1. Andrew Montreal

    Andrew Montreal Well-Known Member Thread Starter

    Location:
    Montreal, Quebec
    Hi all,

    Two questions:

    - I am trying to figure out what effect my Rotel 1592 has on the general tone of my system. To ally speaking, where does it sit?

    Thorens TD125
    Ortofon 2M Bronze
    Rotel 1592
    B&W floorstand (don’t remember the model)

    - How high up would one have to go to upgrade the phono pream?

    Thanks,

    Andrew
     
  2. Andrew Montreal

    Andrew Montreal Well-Known Member Thread Starter

    Location:
    Montreal, Quebec
    *tonally speaking
     
  3. Just Walking

    Just Walking Forum Resident

    Location:
    UK
    The Rotel 1592 has a power amp grafted on to an RC1590 preamp.

    But neither is a clean analog chain. There is an entire digital section, a D-A converter, and soft buttons that control via a processor.

    What actually is the phono stage though? Well no idea; being a current product Rotel are not making available the schematics. So you only have the spec to go on.

    My prejudice is to keep the main items in the audio chain purely analog. Now I do have a streamer - so that is definitely digital, and incorporates a D-A converter of course. But that is the only bit of the system that has any digits to do with it.

    But if I was looking for a first rate phono stage to add on, for not silly money, I'd seriously consider a Lounge LCR MkIII https://www.loungeaudio.com/lcr-series . The owner and designer is a list member on these forums, so you can ask him if it would be a suitable upgrade.
     
  4. keiron99

    keiron99 Forum Resident

    Location:
    Stockport, UK
    Are you saying that the phono stage goes through a digital stage? I'd not heard that before.

    I have a 1592 and it's a solid workhorse. I also have a Sweet Vinyl Sugarcube (click & pop remover). Because the Rotel doesn't have a tape loop in which I could slot the Sugarcube, I had to get an external phono stage. I started with a cheapish Project and could not here any difference between that and the stage within the Rotel. So, I assumed they must both be very ordinary. So I "upgraded" to a Lehmann Black Cube SE II (retails about £850 in the UK). Guess what? It sounded exactly the same as the Rotel and Project. (I should add, this is while bypassing the digital stage of the Sugarcube, but then I also cannot hear any difference whether it's engaged or not.)

    Of course, that's how it sounded to me. 99.9% of people here would disagree!
     
    The Pinhead likes this.
  5. Davey

    Davey NP: Rosali ~ Bite Down (2024)

    Location:
    SF Bay Area, USA
    Below is the phono board they show, think it is the same in a few of the models, just a real simple single opamp design, uses the old and ubiquitous NE5532 dual low noise opamp and a handful of resistors and capacitors. Pretty easy to find something better, threads here all the time about inexpensive phono preamps, but probably doesn't sound bad. Specs don't list the phono input capacitance, but overload is decent at 52mV (presumably at 1kHz).


    [​IMG]
     
    chili555 and timind like this.
  6. Andrew Montreal

    Andrew Montreal Well-Known Member Thread Starter

    Location:
    Montreal, Quebec
    Thanks for the responses.

    I should add that mine is the mki NOT the mkii, should that make a difference.

    I suspect that the phono amp to power amp in the Rotel is purely analog. I would be surprised if it isn’t.

    My reason for considering a separate phono amp is because I like using EQ to balance various recordings out and I happen to have an insane EQ in my studio which could be used for this purpose:

    https://www.sweetwater.com/store/detail/MassivePassive--manley-massive-passive-stereo-tube-eq

    Sure it’s overkill but it’s what I have. And for me EQ’ing is often necessary. Problem is I can’t come out of the Rotel to pass through the EQ before returning to the power amp of the Rotel, therefore an external phono amp is needed… possibly.
     
    Just Walking likes this.
  7. Just Walking

    Just Walking Forum Resident

    Location:
    UK
    That is not what I said. Whatever the analog chain is (phono stage, preamp, power amp) it sits alongside a whole bunch of digital stuff. Fast digital stuff in the same chassis as a phono stage is I would suggest not a good idea.

    But it would seem that Rotel is not unusual in this regard. Quad alas now do the same sort of thing, as do many other audio companies.
     
    Andrew Montreal likes this.
  8. Just Walking

    Just Walking Forum Resident

    Location:
    UK
    Yes I saw that. I really did not want it to be the phono stage! An NE5532 is not the right device. The single NE5534 takes an awful lot of beating for noise - the noise voltage and current match the average impedance of a moving magnet cartridge almost perfectly. But the 5532 - no!

    The OP can do a whole lot better that than that with just about any external phono stage going.
     
  9. Davey

    Davey NP: Rosali ~ Bite Down (2024)

    Location:
    SF Bay Area, USA
    Below is the schematic of the board I posted above ...


    [​IMG]
     
  10. Just Walking

    Just Walking Forum Resident

    Location:
    UK
    Well the spec of the Rotel RIAA amp says 5.2mV sensitivity into 47k. It then says 80dB S/N. They do not mention whether this is with a real world MM cartridge, or with shorted inputs, or a pure resistive load. So who knows what that number actually means.

    For a *noiseless amplifier*, the maximum signal to noise ratio is around 81dB after RIAA EQ ref 5mV. This is a result of a real cartridge into a 47k input R, and no noise from the amplifier.

    Ah - but Rotel sneakily also apply A weighting to the noise before they work out the S/N ratio. That is worth around 3-4dB. So the *actual* S/N ratio, after RIAA and without A weighting is about 76dB.

    That would be about right. Douglas Self in his book Small Signal Audio Design (2nd Ed) models an RIAA stage with different opamps and a typical MM cartridge loading (610 ohms and 470mH), and with an NE5532A he calculates 76.2dB S/N. The NE5534A is slightly more than 2dB quieter at 78.7dB, a worthwhile improvement. Neither figure with A weighting.

    You can do a bit better with a discrete design using low-noise JFETs. Bob Cordell's VinylTrak design, with a MM cartridge connected and no weighting is 80.5dB. Which really is very quiet indeed and almost equal to the performance of a noiseless amplifier.

    Getting down to the question that the OP was asking - is it possible to do better with an external phono stage as compared with the one in the Rotel? Absolutely.
     
    Andrew Montreal likes this.
  11. keiron99

    keiron99 Forum Resident

    Location:
    Stockport, UK
    Don't be ashamed to fiddle with the tone controls!
     
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  12. Just Walking

    Just Walking Forum Resident

    Location:
    UK
    What on earth are those 1k resistors for R451 and R452? Ah - with the 150pF capacitors C451 and C452 it makes a 1MHz cutoff to prevent digital garbage being picked up by the RIAA input. And alas those 1k resistors generate noise.

    How did you get the schematic, by the way?
     
  13. Davey

    Davey NP: Rosali ~ Bite Down (2024)

    Location:
    SF Bay Area, USA
    Just googled the part number on the board in the picture, and first hit showed it listed in the Rotel RA-1062 Technical Manual, so just copied the schematic from there.
     
    Just Walking likes this.
  14. Andrew Montreal

    Andrew Montreal Well-Known Member Thread Starter

    Location:
    Montreal, Quebec
    Haha… I’m definitely not. I’m talking about using $7000 worth of EQ for my record player. No shame here… though of course when needed in the studio it goes back to its place.
     
    Just Walking and keiron99 like this.
  15. Andrew Montreal

    Andrew Montreal Well-Known Member Thread Starter

    Location:
    Montreal, Quebec
    Thank you for the input. It looks like I will eventually upgrade that portion of my setup. I happen to have a Bryston 3B lying around. I might try that as well. Perhaps a phono amp - Manley EQ - Bryston… could be nice.
     
  16. Andrew Montreal

    Andrew Montreal Well-Known Member Thread Starter

    Location:
    Montreal, Quebec
    Well I did it. I went out and bought a stand-alone phono preamp. Nothing too fancy… Someone was selling a Graham Slee SE2 on Marketplace. The difference? Major! No mincing of words here… it was major. Better imaging… better dynamics, especially in the bottom end… that classic veil removal… it was all better.

    I guess most integrated systems have so much to accomplish that they can’t be expected to excel at all tasks.
     
    james likes this.

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