ARP2600 Style Preamp: Understanding and Reducing Noise

Readers: If you want to build the preamp in this post, please go to PCBWAY's Community pages--a gerber ready to download and/or fabricate is here. 

Also please visit PCBWAY's site using the link here--it will help this blog. 

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I'm reaching the end of laying out clones and workalikes based on the classic ARP 2600 synthesizer.  

It's (almost) time to move on.

Not yet! Let's redo the 2600's preamp with revision 2 of a workalike--improving upon the last one--since revision one was noisy and sounded, well, just OK.

Finished Rev 2 Eurorack Synth Preamp

A previous attempt at laying out the same preamp....

I followed some basic guidelines to make a circuit less noisy....to my ears this Revision 2 design is quieter and sounds cleaner; I cover its build in this post; also share my "understanding op amp noise" lab notes.

A $10USD super low noise op amp vs. the venerable 741. Which produces less noise? Hint: the more expensive one. In a future post I will rebuild this post's preamplifier to see if its sounds better still with super op amp; I am using TL072's now.

LAB NOTES--OP AMP NOISE

An immediate takeaway--I thought there was just one kind of noise--but no, there are different types. I will look at voltage noise, current noise, total harmonic distortion, and slew.

VOLTAGE NOISE

Every component in a circuit generates voltage noise—wires, traces, resistors, everything. Temperature, quality of manufacture, IC design, materials used—all contribute to this noise. And of course op amps are components....so they contibute.

We can map op amp voltage noise using a graph:

This is the voltage noise on an OP07 op amp tested under laboratory conditions and shows voltage noise at low frequencies--.1 hz to 10hz.

But we use op amps at all different frequencies; in this case, voltage noise's spectral density looks like this: 

 

Voltage noise that varies depending on frequency is caused by several factors--here I name a few: 

• Johnson noise--present in all components--noise created by thermal agitation of the electrons in the materials used; it spans all frequencies ("white noise") and is most noticable at higher frequencies.
• Shot noise--caused by something to do with quantum mechanics that I can't sum up in a few words and will most likely never fully understand; I opened the box and the cat's dead. Temperature indepedendent; like Johnson noise, it spans all frequencies.
• Flicker noise or "1/f" noise: Noise caused by the materials and construction techniques used during manufacture--1/f noise is concentrated in the lower frequencies--in the graph above you see it from 1 to about 500 hz.  1/f corner is where the white noise and pink noise amplitudes meet.

EEVLOG's video explaining voltage noise, which goes into quite a bit more detail, is here. 

Good articles on the same subject:  here and here.  

An interesting video explaining how to measure 1/f noise--using op amps to measure the noise of other op amps--is here.

From a noise standpoint, how to determine which op amp is right for a project?  

Like online dating: "it depends". 

In the land of AudioDiWHY, we do a lot in the 100-1000hz range, so 1/f noise is not our friend. The comparison chart here shows that the venerable TL072, used for this post's pre-amp build, and my go-to op amp for a lot of what I design, does not compare favorably with many other higher-performance op amps in terms of noise specs.

They're everywhere!

An obvious way to ameliorate: use a lower noise op amp. I got 2 AD8599's for a "final" build whose spectral noise density is ridiculously low, especially at low frequencies:

However, a less expensive op amp like a 5532 might sound just as good, and with my tinnitus diminished hearing I may not be able to tell the difference.

CURRENT NOISE:

There is also another type of noise—“current noise”.  

The idea: an op amp input should source or sink no current, but in reality it does. The current noise can be amplified by feedback resistors and overall gain, meaning  that current noise cannot be ignored. 

Current noise is less of an issue in JFET op amps, since JFETs really are closer to infinitely high input impedance. 

Some videos about this:

https://www.youtube.com/watch?v=1tUaDghBmuc&t=243s

https://www.youtube.com/watch?v=TxBJb-Z0XFI&t=950s

To try to get rid of these issues, there are a few basic design ideas…

For instance: the videos explain why you might see resistors between non-inverting and ground, which never made sense to me until I went down the op amp noise rabbit hole:

The R and C highlighted are there to quiet down the op amp created by input offset bias current offsets. Component choice looks easy--same value resistor as a feedback loop and a small cap.  

Great article about this: https://www.eetimes.com/op-amps-in-small-signal-audio-design-part-1-op-amp-history-properties/

Another basic idea is to use smaller value resistors in feedback loops: see the article here.  

OP AMP SHOOTOUT:

An extremely useful summary for overall op performance: https://www.cycfi.com/projects/six-pack/op-amp-shootout/

It’s from 2017 but many of the op amps are still available.  I bought some OPA1602's I learned about from the shootout; at about $3USD 1602's they have excellent specs for the price. 

THD:   

A bit easier to understand—extra harmonics are created by the amplifier.  A lower number is desirable. Video here.                                               

SLEW:

An op amp cannot keep up with infinitely rapid voltage changes—for instance, a square wave at input does not look as crisp at output on a scope.  A high slew rate is desirable, but too fast an op amp can cause its own problems. Good video about slew is here.

Update 10-5-23: while researching another project I came across the video from Sine Lab here: an easy to understand general summary of op amp performance characterics.

OTHER WAYS TO IMPROVE PREAMP PERFORMANCE 

Ferrite beads and line filters near the power rails can be used to improve general amplification performance.

Using ground pours helps as well. Maybe? For high frequency electronics certainly but for us 20-20K bottom feeders, well, maybe.With my own designs I have not been able to hear a difference, but I read they help.

Again: lower resistance values in negative feedback loops helps. Look here. I reduced the 10K/100K used in rev1 to 1K/10K.  I will use 720ohm/10K for the next build, which takes us closer to 10x gain.

I incorporated these three improvements into this post's "revision 2" preamp.  

LET'S BUILD!

This time I took some of the aforementioned lower noise ideas and put them to the test.  

Here are build photos:

Joy! Bags o' New boards from the blog's trusty sponsor, PCBWAY

Ready to test....

Temporary front panel.... 

Final front panel fabricated.....

With TL072's, not super low noise op amps, the build already sounds better vs. the one here.

As mentioned above, I am going to build another preamp with AD8599's--the $10 op amp lover's op amp--to see if it sounds different/better than the TL072 build. 

Update: dropping in the AD8599, sadly, I can't hear a difference between it and the TL072. But that might be my >60 year old, sonically-tortured-between-ages-15-and-35-years-old ears, my crappy bench B chain, my cheap external Eurorack power supply, or a lot of other factors. For the test to be accurate and useful I'd need to be a lot more scientific, but it's time to move on.  

I'll post a Eurorack format front panel for this circuit as well--shortly. One of these days. Maybe. Update: gerber for front panel is posted--go here.

Anyway, this was a fun build, and it was satisfying to put another piece of  classic ARP inspired tech in my rack.

More to come....stay tuned.