DC Coupled RSIIb Equalizer!

Maxamillion

Super Member
DC Coupled RSIIb Equalizer! No Coupling Caps!

I designed and built a new output board for my RSIIb low frequency EQ unit. Since I am single amping, all the signal passes through this unit, so I want to make it as transparent as possible.

There is a previous thread where I outline all the upgrades I had already done to the EQ, but one thing still bugged me - the output coupling caps. Even though I had upgraded these from electrolytics to a cascade of Mundorf Supreme and Silver/oil, I kept thinking that they were limiting the performance (coupling caps = BAD for sound!). My tube preamp is transformer coupled, my source is DC servo coupled, and my amps have no input or output coupling caps either, so the EQ unit was bugging me!

At first I explored using a transformer instead of the caps, and I talked with Kevin at K&K audio and Per Lundahl, who suggested the LL1585 as an option. I almost pulled the trigger on this since the LL1671s I have in my tube preamp are fantastic, until I talked to Mark Gilmore of Gilmore Audio. I contacted him to ask how much DC his amps could take on their inputs (not much it turns out, <250mV) and he told me that I'd be better off designing a new output stage using opamps, to replace the stock single-ended MOSFET source follower output stage. After I got over my audiophile aversion to opamps I decided to give it a try.

So here's what I did. The new output stage (the tan board in picture #1) consists of an OPA827 single opamp on a Brown Dog adapter (talk about honing your soldering skills! tiny, tiny SMT opamp, but sweet sounding!), wired as a unity gain buffer. I installed a DIP socket on the board so that I can swap in different opamps, just in case I want to try different opamps in the circuit. Picture #2 is a shot of the underside of the board. I added quite a bit of local power supply capacitance for PS decoupling, as the stock unit runs the MOSFETs directly off the regulators. The board uses one 220uF Nichicon Muse KZ per rail (shared between channels) followed by a 10uF Black Gate N (one per rail per channel) and a 0.1uF ceramic (Y5U, one per rail per channel), all between the rail and ground. There is a 470R stopper resistor on the input and a 22R output resistor, both Texas Components TX2575. Before people start ragging me about the "boutique parts", know that all these components other than the OPA827 came from my spares box (yes I've bought nice stuff over the years!). I pulled the 51R, 22R, 470R and 100K resistors from around the MOSFET circuit to disable it - I left the MOSFETs in there as they are totally disconnected after pulling the resistors; no sense adding more work to the job!

How does it sound - in a word FANTASTIC! Crystal clear, with slamming bass and a nice mellow midrange. I listened to it for 12 hours yesterday, and I kept shaking my head in disbelief, it was that much better sounding than stock. It's also much quieter than the stock unit, which had a low level hiss when in circuit that could be heard from about a foot from the speakers; the new unit eliminates that almost completely, most likely due to the greatly improved local power supply for the output circuit. This is as big an upgrade to the overall sound as new Graz diaphragms, and in conjunction with them reveal details that I never knew were there. In fact, the system sounds BETTER with the EQ in the mix than without it! I attribute this to the very low output impedance (~40ohms) and increased current drive of the output buffer, which is able to drive my Gilmore Raptor amps (input impedance of 8kohms) without breaking a sweat - my preamp with its ~400ohms output impedance is greatly helped in this regard by the EQ. No coupling caps = transparency! DC offset was measured at <1mV per channel, so no problems there - be sure to use a low DC offset opamp like the OPA827 in this application, or you will have problems with DC offset on your amps and speakers (remember, the amp multiplies the DC going into it by its gain factor, unless it has a DC blocking cap).

I also put the EQ on the scope. Pictures 3 and 4 are 2kHz and 20kHz square waves - nice and flat, with no trace of oscillation.

Next steps? I might add some local capacitance for the other OPA827s, the ones that add the bass boost. Nothing like a nice fat reservoir of current to draw from! Just need to find the appropriate places to stick some caps from +/- to common.

When I get a chance I'll post a schematic too, in case anyone else wants to try this.


Edit: see post # 4 below re: C0G ceramics - essential for this mod.


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Picture #1 is the schematic for one channel. Picture #2 shows the components removed to disconnect the stock output stage with an X, and also shows where I took the signal and +/-18V from.

In my version I shared the 220uF caps between channels, because I only had two of them on hand, and 110uF per channel should be more than enough capacitance for this application.

After a few days of listening, what I notice most is the increased clarity and detail, especially in the bass. Acoustic bass sounds more realistic, and you can hear the resonances of the strings and the body of the instrument more clearly. Electric bass lines seem to jump out of the background with more clarity. There is also a slight increase in the treble, and I turned down the high tweeter control on the speakers by a small amount to compensate - nothing offensive, just a bit more treble intensity than I like. By the way, I measured the DC offset at the speaker terminals and it's 1.4mV and 5.4mV, well within safe ranges. Note that I left the two 1N4148 diodes (in the stock schematic around where the signal is taken out) in the circuit - those are there to protect the mosfets from overvoltage, and they will perform the same function with the opamps; they do, however leak a little DC, and removing them should lower the DC offset even more.

All in all it's a nice improvement - I'm sure I'll roll in some different opamps over time and see how/if they flavor the sound differently, but for now I'll stick with the OPA827s.


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Update:

The bit of glare in the highs was still present after a couple more days, so I decided to try bypassing between the +/- pins of the opamps. It turns out I just received a batch of 0.1uF C0G/NP0 ceramic caps, so I used those for the bypasses. They are remarkably small for 0.1uF C0G ceramics, and they appear to be dipped in epoxy as well. Here is the Digikey page for them, it seems they are a new product from TDK:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=445-4801-ND

Well, the results were beyond what I expected. The glare completely disappeared! I was so impressed that I immediately replaced the crappy Radio Shack Y5U ceramics I was using for the PS decoupling with the same C0G caps. I have to say, the sound now retains all of the transparency that I love, and also has none of the high-frequency emphasis, in fact it is nicely rounded. Picture # 1 shows the C0G caps across the +/- pins and in the decoupling positions. I also placed these little beasties across the +/- pins of the bass boost opamps as well (picture #2).

I must say - DO NOT do this mod without these caps across the +/-, they are important in achieving the best possible sound. I especially recommend the 445-4801-ND parts from Digikey.
 

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Further EQ Upgrades:

Now that I'm done building the 200WPC SET to drive the RSIIbs (pic #1), it was time to revisit the LFEQ power supply.

The standard supply is a half-wave voltage doubler, and previously I had moved it off the main EQ board and added shielding to reduce noise. Still, though, having AC and nasty half-wave rectifier noise in the same box as the EQ circuit was not optimal. So, I looked through my spares and was able to come up with enough parts to build a really nice dual choke-input (cLC) +/-24.5V outboard supply for the unit - all I needed to buy new was the Hammond steel enclosure and the two 1.5H chokes. Picture #2 shows the assembled supply. The supply is:

0.33uF - 1.5H - 2000uF - 1000uF (in LFEQ box)

There's also a 330ohm-0.022uF snubber between the rectifer bridge and transformer to eliminate ringing and 2.2kohm bleeder resistors from +/- to common to ensure there is always enough current to keep the supply in pure choke input.

This is connected to the LFEQ with an umbilical and connectors I scavenged from a preamp I upgraded a while ago. It all works well. Sound-wise it's a bit quieter than before and I think I can hear a little bit deeper and tighter bass as well.

The transformer I used is WAAAY overkill for this application - it's from an old DBX subwoofer amp and can supply 100X more current than I need for this unit, but it was free, and its higher voltage (32V-0-32V) allowed me to use choke input and still get the required voltages for the EQ!

Rectification is via a surface-mount Schottky diode bridge (tiny black dot in the center of the board).

All in all, a nice little upgrade.
 

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Checked the noise floor of the EQ unit with the new power supply. It's definitely less noisy. Picture is the noise floor with the EQ on and the inputs shorted.
 

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Never one to leave well enough alone, I got to thinking that the EQ was probably still a bottleneck in my system, so I upgraded the output stage opamps from OPA827 to the new 4th generation Burson discrete opamps (see picture). They JUST fit under the top cover.

Sound differences: After a few hours of burn in, the Bursons are noticeably but not hugely smoother than the 827s, with about the same level of detail. The BIG improvement is in the depth and definition of the bass! The Bursons run at 20mA idle current with 10ohms output impedance vs the 827s 5mA and 40ohms - they can really drive the amps hard on the low frequencies while providing noticeably tighter, more detailed bass.

The 4th edition Bursons are also cheaper than the previous - I paid $68 for a pair of them from Parts Connexion. Needless to say, they're staying in the EQ unit.
 

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The 4th edition Bursons are also cheaper than the previous - I paid $68 for a pair of them from Parts Connexion. Needless to say, they're staying in the EQ unit.

How'd you manage a pair of the newly introduced dual channel 4th gen Burson opamps for less than half of what Parts Connexion is currently offering? :D

I'd like to know how the brand new gen 4 compare to the gen 3 in practical use. I have a set in an Audio GD preamp that's part of my computer ambient system with two sets of 6 Kappa attached to the back end (to keep it Infinity pertinent). I've been very satisfied with the improvements the Bursons have provided for the last 3 or 4 years. I won't go so far as to say they sound tube-ish but they made the sound noticeably more vivid and dynamic which is a big plus for a home office ambient system that runs in the background 24/7/365.

Those giant SET amps are still blowing minds. I imagine your rig sounds ridiculously good.
 
what kind of music do you use when you are testing your changes...?

My system is dependent on recording quality...
 
How'd you manage a pair of the newly introduced dual channel 4th gen Burson opamps for less than half of what Parts Connexion is currently offering? :D

I'd like to know how the brand new gen 4 compare to the gen 3 in practical use. I have a set in an Audio GD preamp that's part of my computer ambient system with two sets of 6 Kappa attached to the back end (to keep it Infinity pertinent). I've been very satisfied with the improvements the Bursons have provided for the last 3 or 4 years. I won't go so far as to say they sound tube-ish but they made the sound noticeably more vivid and dynamic which is a big plus for a home office ambient system that runs in the background 24/7/365.

Those giant SET amps are still blowing minds. I imagine your rig sounds ridiculously good.


They are single opamps, one per channel for the output stage I designed for the EQ. The stock unit uses single-ended MOSFETS as source followers for the output; I changed that to a unity gain opamp circuit so that I could completely remove the output coupling caps due to the low (<150uV) DC offset.

The Midlife Crisis amps are sounding incredibly good with the RSIIb speakers. Seemingly unlimited power, along with the delicacy of a 300B SET amp. Addictive.
 
what kind of music do you use when you are testing your changes...?

My system is dependent on recording quality...

I have a playlist of high quality recordings I use to evaluate any changes I make. They range from girl with guitar to hard bop jazz to electric blues to classical. It's all on a hard disk as a separate playlist called "demo". I also use that playlist as a starting point when folks come over for a listen. My player rips the tracks bit for bit from CD onto the 750GB hard drive; after which I can file the CD away as backup.
 
They are single opamps, one per channel for the output stage I designed for the EQ. The stock unit uses single-ended MOSFETS as source followers for the output; I changed that to a unity gain opamp circuit so that I could completely remove the output coupling caps due to the low (<150uV) DC offset.

I saw 2 PCBs on the opamps and was thinking that indicated the dual model. I now realize there's nothing but the pinout on the 2nd PCB. Sorry. :dunno:
 
I saw 2 PCBs on the opamps and was thinking that indicated the dual model. I now realize there's nothing but the pinout on the 2nd PCB. Sorry. :dunno:

No problem. I'm also noticing a blacker background and an almost total lack of glare from the Bursons. Very impressive!

Now, I've read some opamp shootout reports with measurements that claim the discrete opamps are inferior to IC opamps in just about every technical specification, but I'm now convinced that doesn't tell the whole story. I've tried a variety of ICs, and the OPA827 was the best of the lot, but the Bursons just blow them away sound-wise. I suspect the higher idle current has a lot to with that; more time in Class A operation, more drive to the amps, less strain on dynamic passages.
 
Never one to leave well-enough alone, I've changed the high-pass input caps to (2) .01uF Duelund JDM tinned copper caps in parallel on each channel. The 4 Duelunds take up a lot of space, but since I've removed the power supply and output stage from the EQ board there is plenty of room. Sound is clearly better than the Jensen copper foil paper in oil caps I had in there. Enjoying the increased detail and clarity immensely. The .033 Dynamicaps inputting to the bass gain circuit remain as they produce very dynamic, tight and impactful bass.

I also tried swapping the OPA827 bass gain opamps with Sparkos SS3602 discrete opamps, but clearly the BJT-input Sparkos opamps are not compatible with this circuit as they caused a constant rustling/rumbling sound in the speakers. Going back to the FET-input OPA827s removed the noise. Sparkos has great customer service and immediately offered a full refund, so no harm, no foul.

To sum up the latest experiments...Duelund tinned copper input caps were a big improvement, and OPA827 still rules for the bass gain opamps.

Here's a picture of the inside of the LFEQ with the Duelunds and Sparkos opamps installed. Still have the Burson V4 single opamps as the unity-gain output stage.
20210410_110142.jpg
 
If you're not one to leave well enough alone, then you should try the Burson V6 Vivid or V6 Classic opamps. They're supposed to be the cat's meow.

Larry D.
 
If you're not one to leave well enough alone, then you should try the Burson V6 Vivid or V6 Classic opamps. They're supposed to be the cat's meow.

Larry D.

They're on my list to try, along with Staccato, a Polish company. Staccato can run with the existing 18V supply, I'll have to drop to 16V to use the Burson V6s.
On the output I like the Burson V4 because it runs at 18V and at a high current (20mA for a single channel) so it has excellent drive into my amps. The V6s only run at 7mA.
 
OK, so I picked up a pair of the Burson V6 Vivid opamps to swap with the OPA827s in the bass/contour circuit of the EQ (picture 1). I like to bypass the power pins of any opamp I use with at least 0.1uF so I devised a way to do this without cutting open the Burson by nestling a 0.1uF C0G ceramic inside an adapter (conveniently supplied with the Burson!). Seen in pictures 2 and 3. Then I inserted the assembly into the EQ (picture 4). I had to dremel a small divot in the bottom of the Burson case so the cap/adapter could fit right up against the opamp case.

First impressions after 2 hours of listening: the V6 Vivid is indeed very vivid! Bass is at least as good as the OPA827 and a bit cleaner, a bit tighter. The bigger difference is in detail and layering of the sound - backup singers are more easily located and separated from the lead, plucked instruments are a bit more resonant and distinct, and piano sounds more like a real piano. I'd say it's a 10-15% overall improvement, not huge but well worth it.

Barring changes during further use, it looks like the OPA827s have been displaced by the Burson V6 Vivid. The King is dead, long live the King!
 

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One more detail. I exchanged the Belleson +/-18V regulators for Belleson +/-16V SPX78/79 regulators so as not to exceed the max voltage of the Burson opamps.
 
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