DIY Adjustable EQ Phono Stage

the thin end of the wedge
ChrisfromRI
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DIY Adjustable EQ Phono Stage

Post by ChrisfromRI » 07 Oct 2018 02:57

I always wondered whether it might be helpful to be able to tweak the EQ of a phono stage for certain records that seemed to need it - so I decided to build one to experiment with. I put a 6 inch ruler in the front for scale for the front panel picture.

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Here is what the insides look like. Basically it's a MM phono stage using a pair of BB OPA2134 FET Op Amps, with a second stage EQ section using a pair of BB OPA2604 FET Op Amps and Alps pots.

Note that I placed the AC components are at the opposite end of the chassis from the signal circuitry. The chassis is steel, and the transformer is an R-Core with the right windings!

42590

LD100
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Re: DIY Adjustable EQ Phono Stage

Post by LD100 » 07 Oct 2018 04:26

Congratulations, looks like great work! =D> =D> =D>

Luicas
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Re: DIY Adjustable EQ Phono Stage

Post by Luicas » 07 Oct 2018 04:48

Super neat!

Is the phono stage some sort of kit? It all looks very nice indeed.


Luis

nezbleu
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Re: DIY Adjustable EQ Phono Stage

Post by nezbleu » 07 Oct 2018 05:32

Very nice work. Do you have a schematic for the "adjustable" bits?
PS: I think I have one of those exact enclosures, I use it for the raw PSU for my MC phono stage.)

ChrisfromRI
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Re: DIY Adjustable EQ Phono Stage

Post by ChrisfromRI » 07 Oct 2018 06:11

Luicas wrote:Super neat!

Is the phono stage some sort of kit? It all looks very nice indeed.


Luis
Thanks for your kind words. I bought the Phono Stage board and the EQ board already populated with parts from China via eBay, and I bought the BB OPA Op Amps from an honest US supplier I have dealt with regularly so I knew they weren't counterfeit parts, and I just swapped the Op Amps into the on-board sockets.

The Phono Stage board had it's own complete on-board regulated power supply that required the input from a center tapped 36 volt transformer (18 - 0 - 18 VAC). It included two stages of regulation for lower noise.

The EQ board needed to be supplied around 18 VDC, and had its own on-board single stage regulator.

The R-Core transformer I mounted to the side wall had two 117 VAC primaries so could be used in EU at 234 VAC by wiring the primaries in series, or in the US by wiring the primaries in parallel. In addition to its center tapped 36 VAC secondary there were also three individual 9 VAC secondaries available on the transformer, so I wired two of those in series and connected that 18 VAC to the perfboard you can see with the full wave bridge rectifier diodes and smoothing capacitors that I populated onto that perfboard next to the transformer.

The AC Input unit is the type I frequently use that has an integral IEC connector, a fuse, a power switch, and a basic LC line filter in its "can". It's an all-in-one AC input unit and saves time and space.

I mounted the EQ board upside down by only its four pots through the front panel since they were sturdy Alps pots and included washers and nuts. The reason it's upside down is because that was the order I wanted to pots to be in on the front panel (Gain - Bass - Mid - Treble).

The Phono Stage board had an on-board LED so I extended it to the front panel, and I used a resistor decade box to determine the correct brightness of the front panel LED I was using, and that was a 100K current limiting resistor. It was a standard blue LED and I drilled the correct size hole for it and glued it in place with quick set epoxy from the back of the front panel.

The Phono Stage Board and the Perfboard and mounted on standoffs to the floor of the cabinet - which is a steel cabinet style I regularly use and always buy in Black because I can easily label it with my Brother P-Touch using white lettering on black background tape. The knobs and Rhodium plated RCA connectors were on my parts shelf.

If you can't find a specific source PM me and I will send you the link. I don't believe this site's owner likes posting a bunch of links to the broader audience via the Forum.

I have no relation to any of the various parts suppliers, and it seems to sound pretty good even though not broken in or warmed up to any degree.

The main thing I did was keep all of the AC on the right side of the cabinet and as condensed as possible, to avoid coupling any hum into the signal wiring and boards on the left side of the cabinet. It doesn't hum...

ripblade
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Re: DIY Adjustable EQ Phono Stage

Post by ripblade » 08 Oct 2018 00:53

Nice work!

KentT
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Re: DIY Adjustable EQ Phono Stage

Post by KentT » 08 Oct 2018 04:17

A nice idea! How about presets for vintage non-RIAA curves like "AES, NARTB, Original Columbia, FFRR, Acoustic 78, FFRR, New Orthophonic/RIAA, etc. This enhances the utility of this design for vintage record users.

ChrisfromRI
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Re: DIY Adjustable EQ Phono Stage

Post by ChrisfromRI » 08 Oct 2018 06:56

KentT wrote:A nice idea! How about presets for vintage non-RIAA curves like "AES, NARTB, Original Columbia, FFRR, Acoustic 78, FFRR, New Orthophonic/RIAA, etc. This enhances the utility of this design for vintage record users.
I considered that.

KAB already makes some units with many of these precise preset EQ curves built in. My friend has one they made which was completely adjustable and you used it with a four page lookup table based on the year and company on the record label. This was really important for 78s because so many EQ curves were in use. We quickly bored of looking up each and every record label and changing the settings - which BTW didn't always sound right to me even though they were supposed to be perfect.

I made this simpler version to avoid precision/perfection altogether, and just be able to tweak a few knobs until the record sounded right to me - regardless of what EQ they claimed to use. IMO many modern RIAA records need this, despite being RIAA. For example, I have a Frank Sinatra 33 RPM RIAA record that's a greatest hits type record with 20 songs on it, and it needs more bass - as the bass had to get further cut at the mixing board in order to fit 20 songs. Older scratchy records benefit from turning down the treble regardless of the exact EQ curve used. This unit allows me to make personalized correction versus textbook EQ, which is what I decided to go after.

Frankly, if our high-end preamps still had bass and treble controls this would be unnecessary as we could make the correction there - problem is they took those controls away.

ChrisfromRI
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Re: DIY Adjustable EQ Phono Stage

Post by ChrisfromRI » 11 Oct 2018 02:16

I ran a little shootout of moderate priced phono stages and this BB OPA Op Amp phono stage with Tone Controls was better sounding overall than: Primare R20, EAR834 clone, Musical Fidelity V-LPS II, NAIT II clone, generic single 9V battery powered unit.

The shootout system was a nice sounding bedroom system: Denon DP-750/Grace 707/Ortofon D20FL Super with a Primare integrated amp and Mordaunt Short MS-90 speakers.

The only moderate price phono stage that outperformed it was a 14 X AA Alkaline Cell two stage hand matched Toshiba 2SK170 JFET passive EQ (non feedback) unit that I had previously built. This particular battery powered unit had a little bigger soundstage (deeper primarily), better bass extension, better high extension, and some more dynamic contrast due to its super black noise floor. I would say the Adjustable EQ Op Amp unit (FET front end Op Amps) was within at least 90% of this high performing non-adjustable large battery string powered phono stage. Needless to say, I was pleasantly surprised at how good these BB OPA Op Amps sound, and having the adjustable EQ is a real bonus for a lot of recordings.

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Re: DIY Adjustable EQ Phono Stage

Post by smee4 » 13 Oct 2018 02:21

ChrisfromRI wrote:
08 Oct 2018 06:56

Frankly, if our high-end preamps still had bass and treble controls this would be unnecessary as we could make the correction there - problem is they took those controls away.

Exactly right, and they haven't been taken away entirely, even some cheap Chinese units are available with tone controls. Easy to get into an argument about tone controls however. I don't really understand why people hate them when it's easy to bypass or even just set them to neutral, and as you say, they can be very handy.

ChrisfromRI
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Re: DIY Adjustable EQ Phono Stage

Post by ChrisfromRI » 16 Oct 2018 05:19

I loaned this adjustable EQ phono stage to a friend with a very nice sounding and very transparent high end stereo system on Saturday. His is one system I always enjoy listening to, as he is a master of setup and system integration. We're both members of the same audio society so we get to go to meetings and shoot-outs at many other members' homes and we get to hear a lot of quite high end systems, and his is my favorite (next to my own which is just a little different to suit my taste). We spoke tonite, three days later, and I will have to make another adjustable EQ phono stage because he wants to keep that one. According to him the cartridges he had with "too much zing" are now fine and can be used with just a tweak of the treble knob, and his records that needed more bass or less treble are also fine now too that he has the ability to fine tune the phono stage. He offered to buy it at whatever price I felt was reasonable, but I'm hoping we can figure out some sort of trade instead. I guess that project was a success - except that now I have to make another one of them for myself #-o

fscl
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Re: DIY Adjustable EQ Phono Stage

Post by fscl » 17 Oct 2018 17:21

Beautiful =D> =D> =D>

Fred

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Re: DIY Adjustable EQ Phono Stage

Post by sturgus » 18 Oct 2018 02:05

Sweet!!

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Re: DIY Adjustable EQ Phono Stage

Post by cytovette » 16 Nov 2018 15:11

What potentiometers did you use? I have been thinking of doing this on my tube phono preamp. I built it myself, it is a highly modified Hagerman Cornet 2.

ChrisfromRI
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Re: DIY Adjustable EQ Phono Stage

Post by ChrisfromRI » 16 Nov 2018 16:11

They're the smaller body Alps pots, all four are 50K, but not the big body blue velvet Alps pots that cost $15 each.

Since you're adding it to an existing tube phono stage you might have enough extra B+ and heater supply capacity on hand to just add the 2 tube (12AX7) Bandaxall bass/treble board that's available inexpensively on ebay. It's like $20 and you stay all tube.

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