DOA RIAA…

So, the next episode in my Discrete Opamp adventures is an implementation of Bryston’s RIAA circuit based on the DOA33-modules. This is also a circuit that Bryston shows in their published pre-amp manuals and as you can see from the board pictures there aren’t really many components to it beyond the DOA-modules themselves (just like with regular opamps I guess…).

There is no DC-servo in the circuit and therefore it has coupling caps on all inputs and outputs. I’m sure that the coupling caps will scare some people off this design, but to me this isn’t a deal-breaker. The whole project is more about exploring the modular DOA-concept and less about building the ultimate reference RIAA anyway.

There are a couple of different versions of the schematic available from Bryston with only minor differences in parts values. I picked one and stuck with it, but my guess is that there are small audible difference to be explored if you want to. My component values are otherwise pretty much aligned with the schematic and the few exceptions (the input capacitance and the load capacitance) where the changes are either not significant (size of input cap) or improvements with my equipment (load capacitance).

Now, as you can probably glean from the pictures I haven’t actually made the four DOA modules that go on to the RIAA boards yet. However, I have been able to confirm that the board is working with the two modules I have on hand already and so up next is getting a quad of DOAs (probably the SMD-version this time) built up for a proper listening test 🙂

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Project files: The RJM Emerald RIAA

Last week I showed my version of the “Emerald” RIAA design by Richard J. Murdey. The Emerald is a neat little design: It has switchable gain and load for MM/MC, if you use good components it’s got a very accurate RIAA-curve, and of course with just two opamps per channel as the active devices, it’s very easy to build.

Richard has graciously shared the Eagle-files for his version and so it seems only right that I do the same here. Richard is also selling boards from his website, so if you want something that is proven to be working and comes with support then I suggest you buy your boards from him instead.

A real gem…

I’ve already had my hands on a couple of designs by Richard J. Murdey, mainly here and here. However, Richard has quite a few other interesting designs on his site and one of them I’ve had my eyes on for a while, namely the “Sapphire” which is a line/headphone amplifier. Unfortunately my first order of Sapphire boards never showed up and while I was working on the Sapphire I also had a look at the Emerald MM/MC RIAA. Those boards did show up and so while I wait for the reordered Sapphire boards to (hopefully) show up soon, let’s look at the Emerald instead 🙂

The Emerald is very simple RIAA-design with two opamps per channel and selectable gain for both MM and MC cartridges. The RIAA correction is a bit special as described in Richards write-up and the design also includes an on-board discrete voltage regulator.

Richard has graciously shared the Eagle files for his designs so that was what I started from, but what was originally meant to be minor touch-ups ended up taking me a bit further away from the original than I expected. Although the schematic is still (more or less) the same, I’ve made quite a few changes to the board so that it now looks more like one of mine and uses the parts footprints that I normally use – the latter is absolutely intentional, the former a bit less so. I’ve also done quite a bit of rerouting – which I think of as optimizing the layout, but others might disagree – and managed to shrink the overall board dimensions a little in the process as well.

I haven’t really sacrificed anything from the original, except maybe by downsizing the footprint for the output cap a little bit, but that’s hopefully an acceptable compromise for most people. Project files (probably) coming shortly 😀

Project files: The Muffsy RIAA clone

What is it?
My version of the “Muffsy” RIAA board. As I observed in the first post my version really looks nothing like the original, so here’s a quick rundown of what I have changed:

• The board is mono and intended to be stacked two channels on top of each other as shown in the pictures because that was what I needed. Side-by-side placement and/or dual-mono configurations are of course possible as well – if maybe a little less practical…
• The board uses single opamps instead of the duals on the original. This of course increases the cost a bit, but it also offers more flexibility for those that want to experiment with how different opamps change the sound (because the choice of singles is much bigger – especially after this).
• Because I’ve found myself using the mute button on my amplifier quite a lot when changing records, I got the idea to include a relay-based mute on the output of the RIAA so that mute is available even if your amplifier doesn’t have a mute option.
• The input cap position has been kept because my usual cartridge expects a slightly higher load capacitance so I need a capacitor in that position (you can of course still leave it out if you want).
• The output cap position is bigger to make space for a more “audiophile” sized capacitor 🙂
• The selectable input impedance resistors have been removed. All of the cartridges I am reasonably going to use on my Pro-Ject turntable will be MM-types that need a 47k load impedance so I skipped this option – not that it’s a bad idea though.
• The selectable gain I have kept, but instead of a DIP-switch I used a header/jumper which doesn’t take up as much space on the board. It’s only marginally less user-friendly than the DIP-switch in my opinion.
• Otherwise the basic schematic and the component values remain the same as the original Muffsy.

How big are the boards?
The board measures 3.925” x 1.45” (app. 100 x 37 mm.).

What is the status of the boards?
The board file is v1.1 as I’ve fixed some minor niggles with the prototypes. I’ve tested the board and it works but I still don’t have detailed listening impressions, not least because – as the pictures show – I’m missing a couple of fairly important PSU components 😉

Does it use any special/expensive/hard-to-find parts?
Most of these components are standard sizes and it should be easy to find from the usual sources, however note the following:

• Capacitors in the signal path should be polypropylene** film types and ideally 2% tolerance or less and in packages with 5mm pin spacing. If you can’t find 2% capacitors, buy a few 5% types of each value and match them using a multimeter to at least keep the variance between channels as low as possible (honestly this is worth checking regardless of what you buy).
• Resistors should be 1% tolerance (or better – so if you want to “splurge” on 0.1% types go right ahead 😀 ). Obviously the resistors for the relay and the LED are exempted from this requirement 🙂
• The muting relay is a tiny G6K type from Omron and as far as I know there are no substitutes from other manufacturers. Make sure you get the version with even 0.1” pin spacing.
• I don’t really recommend buying expensive opamps from eBay or aliexpress because it is simply too easy to fake the markings on a standard DIP-package to make a surplus TL071 look like a NOS OPA627 at 10-20 times the price, but only you can decide if you think the risk is worth taking. There are some great offers on eBay, but sometimes you have to go through quite a bit of grief to find them…
• **No, in actual fact no one is really going to die because you use polyester film caps but you’re an audiophile so you should aim for the best (within reason, if you happen to be both an audiophile and an engineer or just an audiophile that still has a bit of common sense left 😀 )

Anything else I need to know?

• The muting relay is powered by the positive supply rail (via a resistor if the voltage is higher than 12V) with the switch in series, so any latching switch will be usable as the mute switch.
• As standard, the muting relay switches both signal and ground. Not sure if this is always a good idea, but if not then it’s quite easy to bridge the GND-connection on the back of the board so only the signal is switched.
• If you don’t want the mute option at all just bridge both of the positions marked on the back the board and you can skip the relay, the resistor, the diode across the relay and of course the connector and switch.
• As is my custom for PCB layouts this is quite tight (ok, make that “cramped”), so a couple of tips: 1) remember the decoupling caps on top and 2) measure the voltage on the opamp sockets to ensure that everything is OK before mounting the opamps becasuse the decoupling electrolytics make it a bit difficult to remove the IC afterwards. If you want to experiment with different opamps, consider stacking two sockets on top of each other. This should lift the actual opamp above the electrolytics making it easier to remove.
• Again: Note the two SMD decoupling caps on the top of the board. They need to be mounted before you solder the opamp sockets!!. They also mean that you have to use a socket and you can’t mount the opamp directly to the board.
• I’ve used a basic LM317/LM337 power supply for mine (this one to be specific), but there might be something to gain by using more sophisticated low-noise types. It is also possible to power the circuit from a couple of 12V SLA batteries, but if it is worth the trouble I’m not sure (probably not though – if you want to put that much effort into a RIAA you should maybe consider starting from a more advanced circuit instead 😀 ).
• Regarding the opamps, the “default” option is the LME49710, but there are lots of other DIP-options as well that could potentially be used (LT1363, LT1028, AD797, OPA627 etc. are all common choices for RIAA-circuits) as well as of course several others in SO-8 packages that can be used with suitable adapters. Unless you already have a favourite IC, my advice would be to start from the default (which should be among the best “value-for-money” options anyway) and then change once you’ve tried it so you can more easily identify changes.

Downloads:
Download design files here

Related information:
As this is effectively a clone of the Muffsy board, you should read the Muffsy website for additional information (and if you really want the background, the original Audiokarma CNC thread as well).

Note: Always read the “intro post” for additional important information about my designs.

Oh, and remember I did this because I wanted to experiment and tailor an existing design to my own requirements and preferences. If you just want to build a board and get sound out of it, don’t bother with this but buy boards/kits of the original Muffsy instead 😀

The Muffsy RIAA clone (teaser…)

It’s been a while and I’ve got no time to do a big writeup right now (it’s Christmas eve!!) but it’s here, it’s working and no, it looks nothing like the original. More in a few days but a very merry Christmas to everyone for now 😀

Phew!

Been very busy lately, with both life in general and (especially) a job change. So, finally it’s been a (relatively) free weekend and a chance to relax with some solder fumes. Haven’t build anything that’s worth showing off at the moment, but it was nice to have some time to build again 😀

Apart from a bit of soldering I also managed to finish some PCB files as well. Most important was the PCB for my version of the Audiokarma CNC RIAA (or, actually the “Muffsy” evolution version of it). I’ve been listening to a lot of vinyl lately and this looks like a very interesting (and simple) RIAA, so I had to give it a try – albeit with a few changes over the original. Boards should be here in a couple of weeks 🙂

Project files: The Bas***ds

What is it?
These are the files for my revised version of the “The Bastard” preamp and RIAA design (see blog posts here and here)

How big are the boards?
– Line stage: 2.15″ x 3.90″ (app. 55 x 99 mm)
– RIAA: 3.0″ x 3.875″ (app. 76 x 98 mm)

What is the status of the boards?
The boards are both in version 1.1. Version 1.0 has been prototyped and version 1.1 incorporates some minor tweaks and fixes to the layouts.

Does it use any special/expensive/hard-to-find parts?
Not really. The 6J5 tubes should be possible to find on ebay and similar places. There is also a 12J5 version which, apart from needing a 12VDC filament voltage, should be identical to the 6J5. As for other substitutes, please search online. The caps in the line stage and the RIAA are Mundorf standard MKP types (the white ones).

Anything else I need to know?
Not really, no. The boards take a +37VDC (or thereabouts) main power supply and draw very little power – you can easily use one of the regulated PSUs I have posted here on the site. The Line Stage additionally needs a +6.3VDC filament supply. Current draw depends on the type of tube chosen, but you can use one of the regulated PSU designs as well as long as you use a 6 or 7VAC transformer.

Downloads:
Download design files here

Related information:
Note: Always read the “intro post” for additional important information about my designs.

Link to the original design/build article (in Danish) is here (20MB scanned pdf).

A real Ba****d – Part 2

Second part of my “rework” of the old “The Bastard” – the RIAA-stage. The first was the tube-based line stage which you can see in one of my previous posts and there you can also find a link to the original article from a (long gone) Danish magazine.

This is a discrete solid-state RIAA stage based on fairly standard BC5xx-transistors. I know virtually nothing about RIAA-designs, toplogies, correction curves etc. so I will not comment on that and all I have done is to lay out a new PCB in Eagle. My enclosure parts for this build are currently on order, so I have only managed to listen very briefly to the board but it does sound promising I have to say :). Once it is built I will be listening further and comparing it against the Retro as well (in an “old-versus-new-technology” shootout :D).

The “Retro” balanced RIAA stage

Since I got my Pro-Ject turntable about two months ago I have been looking for a decent RIAA-design. I have an old Vincent, but it isn’t really good enough. Almost at the same time, I realized that Brian and Russ at Twisted Pear Audio were just finishing up a balanced RIAA design using differential opamps which they called the Retro. It looked like it might be worth a try, so I ordered some PCBs and the few parts that I were missing – sometimes it is nice to have a big cache of parts that can be used 😀

The build is dual-mono including power supply boards (the LCBPS also from Twisted Pear) and separate PCB-mounted transformers for each channel. It is a bit overkill with two transformers, but hey, they were just laying around anyway 😉 The observant reader will notice from the pictures that my ground wiring isn’t 100% correct – I am still trying to get my head around grounding in differential systems but for now it works fine with only a little hum in the speakers.

I haven’t spent two much time listening yet, but even with short tests it is obvious that this design is at least a few steps up from the Vincent, so although I have a few more RIAA-designs I would like to try as well, the Retro is definitely making the Pro-Ject (and the vinyl collection) sing 🙂