HF-81 Phono Section

Discussion in 'Eico Place' started by silversport, Jul 26, 2017.

  1. silversport

    silversport Super Member

    Messages:
    2,323
    Location:
    ChicagoLand-USA
    just listening to my EICO HF-81 phono section for the first time...I am such an idiot... [​IMG]

    years ago, Craig at NOSValves rebuilt my EICO HF-81 integrated amplifier...I seem to recall Craig asking me something about if I wanted something done to the phono section but a TT was less important to me then...years pass and I have an interest in listening to my records from back when...

    Having sold off other electronics that would allow me to use a TT my younger brother stepped up and let me use an unloved Nakamichi SR-2A STASIS receiver to give it a whirl again...it's been okay in this regard but the Nad needs some love to get back in spec so the experience is lacking...then it hit me...the EICO I keep in an upstairs bedroom system has a phono section I've never used or tried...

    Unhooked the EICO and took it downstairs, hooked up my Technics SL-1300 with a Shure VF-15 Type lll...grounded the TT to the EICO chassis and...it's not bad in fact it sounds quite good...and it is better than the aged NAK...

    I think I always thought since I had told Craig the Phono section was not important to me that it might not be even connected...don't know why I did but there it is...

    I just thought I'd share...

    Bill
     
  2. tubeactive

    tubeactive AK Subscriber Subscriber

    Messages:
    11
    Actually, the stock, factory phono circuit is a good one. You hear it that way, which is a good sign, especially considering that phono circuitry may still have 1958-1959 parts in use ! You can "optimize" that circuit with fresh, tightly matched parts. Phono RIAA EQ of this "active feedback EQ" type contain two EQ networks in series; one is for the Bass Boost/Turnover and the other is for Treble Rolloff. Ceramic caps were used in the original phono EQ wiring. The 2700 pf "Bass EQ" caps are worthy of becoming polystyrene, mica, polypropylene or mylar. They are in parallel/across the 2.2 Megohm resistors.

    The 850 pf "Treble EQ" caps can benefit from similar parts' type upgrading, but they can actually be lowered in value, for better RIAA EQ adherence. First, measure the 90K Ohm resistors in parallel with these 850 pf caps. They should read similarly. Over time, they usually "drift" to a higher value, like 92K or 91K Ohm if you are lucky. Phono EQ networks, of R-C type (resistor-capacitor), each have a time constant. Proper RIAA Treble Rolloff is 75 uSec. That means the R x C = 75 uSeconds. 90,000 x .000850 = 76.5 uSec which is a bit high. If the 90K ohm resistors have become higher in value, we can lower the 850 pf value. Let's say the 90K read 92K with a good meter. 92,000 Ohms x .000815(815 pf) = 75 uSec. 91,463 Ohms x .000820 = 75 uSec. 820 pf is a standard cap value. If the treble EQ resistors still read close to 90K Ohms, 833 pf is called for, not 850 pf as in the original.

    Eico actually adhered to the proper RIAA EQ math formulae. Thus, when they supplied the parts in their kits, the 850 pf caps they supplied may have actually measured closer to the 833 pf. If you look at their original plate load resistors, usually 5% tolerance printed on their sides, Eico chose closely matched, low-noise resistors for these parts.

    If you or anyone else studies the Eico HF-81 or HF-85 schematics, you will eventually realize that there are three "low level" (high-gain) stages included in these units. They are marked Mic, Phono and Tape (Head) on the fascia. If you can decipher the circuitry and are electronically savvy, you can convert the Mic and Tape inputs into additional Phono inputs. Furthermore, those usually unused Tape EQ switches on the face, marked 3 3/4 and 7 1/2, can be converted to switchable treble rolloff phono EQ network circuitry. For those of us with two or three turntables nearby, or having some early 1950s mono discs, these Eicos can sound stellar with some upgrading...

    Enjoy your HF-81 and Keep On Groovin"...........
     
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  3. silversport

    silversport Super Member

    Messages:
    2,323
    Location:
    ChicagoLand-USA
    thank you SO much for your response!...

    Bill
     
  4. silversport

    silversport Super Member

    Messages:
    2,323
    Location:
    ChicagoLand-USA
    seems I prefer the EICO with Quartets rather than the RB-5s...I also prefer the EICO with Heresys and Chorus lls...

    Good luck with your choice.

    Bill
     

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