A Dual Path RIAA EQ/Monitor Switcher for RAW Transfers

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A Dual Path RIAA EQ/Monitor Switcher for RAW Transfers

Unread postby Wayne Kirkwood » Thu Aug 13, 2015 4:17 pm

This is part two of the Flat Phono Preamp for RAW Transfers series that describes the RIAA EQ/Monitor Switcher "mult-tasker."

Part 1, the Flat Balanced Input/Balanced Output MM preamp, is here: viewtopic.php?f=9&t=5914

Background from the previous thread:

For a number of years I've been working on a moving magnet preamp for flat, "no RIAA EQ applied" RAW recording. Flat recording gives de-clicking software a far better capture to work with. Once de-clicked, the files can be rendered with RIAA in DSP.

One of the most obvious limitations doing flat transfers is that monitoring the transfer is painful to listen to. When de-clicked in the un-EQ'd, RAW domain, de-clicking adjustments cannot be auditioned and heard in the proper, RIAA de-emphasized, context. With a flat preamp and DSP-based RIAA one still needs analog RIAA EQ. For general listening or needle drops having de-emphasis is an imperative.

The ability to simultaneously record and playback at the same time is a huge time saver. Though transfers do need to be confidence monitored during recording, one doesn't have to listen to an entire side. Being able to edit, de-click and render RIAA while recording enables true multi-tasking. For this reason I designed the flat transfer system to have separate record and monitor paths.


Flexibility is King

I tried to put in a lot of flexibility and make the RIAA EQ/Monitor switcher a "Swiss Army knife."
Paul Gold's suggestions, particularly the inverse RIAA EQ for dub plate cutting, really helped.

Though it looks complicated it doesn't have to be.
Someone wanting a simple RIAA EQ'd phono preamp needs only to build that part of the board.
My application, for LP archiving using RAW flat transfer, requires the full circuitry.

Schematic for the RIAA EQ/Monitor switcher:

Image
RIAA EQ, Inverse RIAA EQ and Monitor Switcher for the Flat Phono Preamp.

Signal Flow

The design has two signal paths. One is for recording the second for confidence monitoring or simultaneous recording and playback.
An RIAA EQ section can be placed in either the record path or playback path.

Flat transfers can be made through the record path while non-destructively switching monitor modes from monitoring flat record, record with EQ or playback from the DAW output.
Having analog RIAA EQ for monitoring saves ears and permits later de-clicking, done in the RAW un-EQ'd domain, to be heard in context.
Final EQ of the transfered files is done in DSP.

The flat balanced input/balanced output moving magnet preamp can be remotely located and powered from the monitor switcher permitting long cables between the turntable and workstation.
(See: viewtopic.php?f=9&t=5914)

The flat/wideband preamp output is received by a THAT1240 line receiver.
It is then AC-coupled into a variable gain inverting buffer.
Relay RY2 selects either flat or RIAA EQ recording mode.
(RY1 must also be in Record for RIAA de-emphasized recording.)
The operation of RY1 and RY2 are destructive.
The record out is available as an RCA connection or fully-balanced by a THAT1646.

The RIAA EQ section can be switched between the Record and Playback Monitor paths by RY1.
A post-EQ gain stage can be used to provide stepped gain trim and additional post-EQ gain if needed.

The DAW playback output is fed into the THAT1246-based line receiver.
RY3 selects either Flat or EQ'd monitoring from either the Play line input, Preamp Output or RIAA EQ output.
The operation of Monitor relay RY3 is non-destructive to recording.

Jumper Description

J1 bypasses the balanced input stage. This is used when the flat preamp is in the same chassis as the EQ/Monitor Switcher.
When J1 is connected directly to the input, the THAT1240 on the EQ board and one THAT1240 on the flat preamp may be removed.

J2 (optional) permits the preamp gain to be raised or lowered in 3dB steps. It can also be a front panel switch.

J3 is a rumble/warp filter insert.

J4 is for alternate EQ. It can be used to bypass the RIAA stage and for inverse RIAA dub cutting allow the insert of a 50 kHz ultrasonic filter. (e.g. Neumann 2 pole SK)

J5A/J5B switch the RIAA filter to inverse RIAA to permit the board to be used for cutting discs.

J6 is an optional cartridge correction filter insert point. J6 may also be used to provide a return for alternate EQ.

J7 permits the post-EQ gain to be switched in 3 dB steps. J7 can also be used to sum cutterhead feedback with modulation.

J8 is a second playback cartridge correction filter insert. J8 can also be used for additional cutterhead EQ.

J9 selects the flat monitoring source. J9 can either be connected to the Playback line input or it can follow the RIAA EQ. J9 can also be made a switch. For transfers which are going to primarily be made without RIAA EQ, "RAW," then J9 should be jumpered to follow the RIAA EQ. If RIAA EQ'd transfers are the norm, placing the RIAA EQ in the record path, J9 should be jumpered to Play line in to permit confidence monitoring or simultaneous editing of files already recorded with EQ.

Board Stuffing Options

There are lots of them but these are the highlights.

The preamp gain after J2 can have an optional trim pot installed.
Gain is typically optimized and adjusted in the flat moving magnet preamp.
For moving coil preamps, fine adjustment of gain is not always possible.
A trim after J2 allows additional "helper" flat gain and adjustment of channel balance for moving coil carts.

If the flat preamp is co-located with the EQ/Monitor switcher, the THAT1240 balanced input is not required.
The flat preamp can also have one THAT1240 removed.
Due to signal inversion, the inverting output of the flat preamp and inverting input of the EQ board are used to maintain correct polarity.
This is also the reason the THAT1240 appears "upside down" with the + input connected to the - THAT1240 input.

The RIAA EQ shows optional capacitors bridging the 10 nF.
The 10 nF do not require them.
If precise 1% 10 nF are not available, a combination of 6800 and 1500 pF can be stuffed.
The resistor values then become 11K8, 31K8 and 318K with links in the unused series resistor locations.
This combination also hits the 75, 318 and 3180 us time constants almost exactly.
(For more information on the unique Robert's P10 EQ see: viewtopic.php?f=9&t=5848)

For applications which do not require a post-EQ gain trim it may be eliminated and a fixed resistor installed.

Conclusion:

Two simultaneous signal paths allow flat transfers to take place while other files are de-clicked, edited and rendered with RIAA EQ in DSP.
Having two paths is a huge time saver for archival transfer because it allows two things to be done at once.

There will be no cigarette breaks for interns doing transfers with this box - if you got time to lean (waiting for a side to finish recording) you've got time to edit.
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Wayne Kirkwood
 
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Re: A Dual Path RIAA EQ/Monitor Switcher for RAW Transfers

Unread postby Wayne Kirkwood » Wed Aug 19, 2015 2:34 pm

I did a drawing of the proposed front panel to help visualize the switching and routing options.

Image
RIAA EQ and Monitor Switcher Front Panel Switches for the Flat Phono Preamp

The switches from left to right operate RY1, RY2 and RY3.
The fourth switch, which is optional, is at J9.
J9 is used primarily when RIAA EQ is used during recording and flat playback from the line input is needed for simultaneous record and playback.

Image
RIAA EQ, Inverse RIAA EQ and Monitor Switcher for the Flat Phono Preamp.
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Wayne Kirkwood
 
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Location: Dallas, Texas

Re: A Dual Path RIAA EQ/Monitor Switcher for RAW Transfers

Unread postby Wayne Kirkwood » Thu Aug 27, 2015 5:29 pm

Got the pre-production run of boards in for the RIAA EQ/Monitor Switcher and they checkout OK.
The board is a Swiss Army Knife capable of also doing line-level Inverse RIAA.


Image
RIAA EQ/Flat Dual Path Monitor Switcher for the Flat/RAW Balanced Input Phono Preamp

The THAT1240 balanced inputs from the remote flat preamp are on the left.
In the middle of the group of input connectors is the DC power feed to the preamp.
The THAT1246 Line Inputs are the bottom two connectors.

The Flat gain is adjustable by VR1 and VR2.
John Roberts' P10 RIAA EQ stage is in the middle between the relays.
VR3 and VR4 trim the post-EQ level.

RY1-RY4 switch the various monitor and EQ functions.

The THAT1646 balanced outputs are on the right.
Secondary -6 dB unbalanced outputs are also available for RCA or oscilloscope monitoring.

There are no electrolytics in the primary signal path.
(The capacitors around the THAT1646 are for OutSmarts and are essentially out-of-circuit for balanced outputs.)
The unbalanced outputs pass through 10uF bipolar electrolytics.

Jumpers:

J1 bypasses the input THAT1240 if the flat preamp is located with the EQ board.
J2 adjusts the Phono input level in 3 dB steps. (Can be made a FP switch.)
J3 is an insert at the Phono post-gain trim stage.
J4 is a pre-EQ insert.
J5A/J5B can reverse the RIAA EQ to provide Inverse RIAA EQ for cutting.
J6 is a post-EQ insert.
J7 adjusts the Post-EQ level in 3 dB steps. (Also can be made a switch.)
J8 is an insert at the RIAA EQ post-gain trim stage.

I now have all the pieces: (1) Balanced tonearm cable, (2) Flat Balanced Input Moving Magnet Preamp and (3) the RIAA EQ/Monitor Switcher needed for my Phono Transfer Rig.
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Wayne Kirkwood
 
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Location: Dallas, Texas

Re: A Dual Path RIAA EQ/Monitor Switcher for RAW Transfers

Unread postby Wayne Kirkwood » Sat Nov 07, 2015 2:27 pm

Hi trolls - I wanted to let people know that I now have PC boards for the Phono Transfer System in stock which include both the flat preamp and the RIAA EQ/Monitor Switcher board.

I don't have the web page built for them on the http://www.ka-electronics.com site but the ebay listings for them may be found here: http://www.ebay.com/cln/wayne2646/audio-mastering-tools-from-wayne-kirkwood/173444429010

The construction information is on the Pro Audio Design Forum http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=7&t=753

Image
Phono Transfer System with a Flat Phono Preamp and RIAA EQ/Monitor Switcher
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Wayne Kirkwood
 
Posts: 29
Joined: Tue Jul 21, 2015 12:43 pm
Location: Dallas, Texas


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