Super Stock Eico HF-85

[Sam Cogley]

As seen in xbart's thread, I realized that I had a major brain cramp with the way the three low-level inputs function, and some adjustments need to be made to the schematic to get them to work right when converting low-level inputs to line-level.

Quoting myself from that thread:

I've been looking at this again, and just noticed something about the selector switch. My "Tape Head" high level input won't work as designed. If you only want "Phono" to be a low level input running through V1 and V2, connected the lead that would go to terminal 7 on layers 1D and 1E to terminal 1 on those layers, with the other wiring. Leave 7 open. This wonky switch uses terminals 1, 2, 12, and 7 to apply the equalization feedback signal to the output of the anode connected to pin 1 of V1 and V2. It looks like you can have 1 or 3 low-level inputs, but not 2. There may be a workaround, I'll have to scratch my head a bit more.

Workaround for 2 low level inputs, all RIAA equalized: remove the wire connected to terminal 7 of 1C/1D (1E/1F). Connect the RIAA components between pin 2 of V1 and the connection previously connected to terminal 7 of 1C/1D (1E/1F). Make a new connection from there to terminal 11 of 1B (1G). Connect terminal 12 of 1B (1G) to terminal 12 of 1C/1D (1E/1F). Connect terminal 3 of 1B (1G) to terminal 1 of 1C/1D (1E/1F). There should be nothing connected to terminals 1 and 6 of 1B (1G), and terminal 7 of 1C/1D (1E/1F) at this point.

http://audiokarma.org/forums/index.php?threads/eico-hf-85.684585/

Revised schematics will be forthcoming at some point.

 

[Sam Cogley]

Finally thought about fixing this.

2 Phono inputs (Mic converted to phono, Tape Head converted to line-level):

1 Phono input (Mic and Tape Head converted to line-level):

Both versions are drawn out with seven inputs, which requires two HF-81/85 input switches to build one switch. Otherwise the circuit will retain the stock 1.5 input setup around the AM-FM and FM Multi inputs. Phono loading resistors are based on common values, they can be substituted for whatever value the user desires. The switch marked "Phono DPDT" is the former tape equalization switch on the front panel.

 

[2526]

Cool project.

 

[Sam Cogley]

After all this time, I'm finally getting ready to start building on this thing. Some final revisions to the schematics - cleaned up the depiction of the variable phono loading switch, and added the "floating potential" heater system from the ST-84.

One phono, all other inputs line level:

Two phono (Mic and Mag Phono), all other inputs line level:

If one wanted to, a 4PDT slide switch could be sourced, and the second phono input could also have variable loading. Still not quite as cool as the Knight preamp that used a dual pot to achieve infinitely variable cartridge loading, but that would require a bit more surgery. This system was just designed to take advantage of a now-useless switch.

And now I have the truly crazy thought to make a new brass faceplate for this thing, relettered to better reflect the current functionality. Though if I'm going to do that, it would make more sense to use the MIC input as the last line level (labeled as "TAPE" and use the spot now labeled "TAPE" as "PHONO 2," so all of the line level inputs are together.

 

[Sam Cogley]

Good grief. I should probably build this thing before I hit the 10th anniversary of that last post. Life has been weird during that time...

 

[tubeactive]

Here is what I did with mine, as well as my HF-81. I kept the tube rectifier, but doubled or tripled capactances, then split the phono stage B+ into two rails, one for each channel.

The tube rectifier is more than adequate and lends the sweet sound. The AC filaments also sound sweeter than DC fils, imho.

I made the Mic and Tape Head inputs into phono inputs. The Tape Head switch gave me two different phono playback EQ curves. The useless Loudness switch became a Scratch Filter. This displaced my reference KT-600 and Paragon E ten years ago.

It's a long read, but has informative phono playback info...

https://www.lencoheaven.net/forum/index.php?topic=25066.0

 

[Sam Cogley]

Here is what I did with mine, as well as my HF-81. I kept the tube rectifier, but doubled or tripled capactances, then split the phono stage B+ into two rails, one for each channel.

The tube rectifier is more than adequate and lends the sweet sound. The AC filaments also sound sweeter than DC fils, imho.

I made the Mic and Tape Head inputs into phono inputs. The Tape Head switch gave me two different phono playback EQ curves. The useless Loudness switch became a Scratch Filter. This displaced my reference KT-600 and Paragon E ten years ago.

It's a long read, but has informative phono playback info...

https://www.lencoheaven.net/forum/index.php?topic=25066.0

The dangers of sidetracked projects - you've given me a few ideas that I may implement here. Unfortunately, that makes me really want to create a new faceplate - I plan to keep the loudness circuit based dcgillespie's suggestions to rework it along the lines of the Dynaco PAS loudness circuit. Implementing the different EQs on the Tape selector switch instead of the variable phono loading that I had planned is a great idea, as is the scratch filter - which means I need four slide switches instead of three. Thinking about setting it up so that PHONO 1 is regular RIAA, and PHONO 2 has the older NAB/RIAA and AES curves switchable.

 

[tubeactive]

Excellent ! Planning is good...I am glad my essay inspired you. The HF-85 has great potential and has become my primary pre.

Don't forget about the Mic stage. Stock, it is a useless, high gain input.
The one phono EQ I would have liked to implement, is the 800Hz Turnover, with 12-16 db Rolloff. Early 78s need more mid bass and midrange gain. Acoustic 78s can sound way too dull. Plus, I think I want the Scratch Filter to have a sharper cutoff slope.

Maybe, I will get to my second HF-85, this decade...

Enjoy your retro-resto !

 

[Sam Cogley]

Excellent ! Planning is good...I am glad my essay inspired you. The HF-85 has great potential and has become my primary pre.

Don't forget about the Mic stage. Stock, it is a useless, high gain input.
The one phono EQ I would have liked to implement, is the 800Hz Turnover, with 12-16 db Rolloff. Early 78s need more mid bass and midrange gain. Acoustic 78s can sound way too dull. Plus, I think I want the Scratch Filter to have a sharper cutoff slope.

Maybe, I will get to my second HF-85, this decade...

Enjoy your retro-resto !

If you look at the schematic I worked up a decade ago, one of the phono inputs is the old mic input. The Tape input is turned into a high level input labeled “tape.” Building one this way requires a new jack plate for four pairs of RCA jacks (the one I’m using is chopped out of a the back of a scrapped Fisher 800B chassis) and two HF-85 input switches to build one, splitting the weird 1.5 tuner input into two separate inputs. The other option would be a new switch and building a relay board to switch inputs - something that I’ve also considered. Another option that I’ve given some thought to is working things to eliminate one high-level input, and having two RIAA inputs. A position next to one of the RIAA inputs would be connected to the same set of jacks and run the older EQs. Decisions, decisions.