Opamp for fx loop

[chunkitup]

I am looking at opamps that run on a heater circuit rectified. Here is what I was looking at:
NJR 4558 Operational Amplifiers - Op Amps | Mouser
Please let me know if these are good candidates and will running lower voltages affect sound quality. A friend loves the fx loop from the PV delta blues and I was gonna see how it sounds in a sloclone.

[The Dude]

That should work fine. The 5532 would be lower noise. There are also companies that have kits like THIS ONE and others.

[flyingdutchman]

The 5532 would be lower noise.

The low noise characteristics of the 5532 will only make a difference if the opamp is used in low impedance circuitry. Very quiet circuits can be built with this opamp, but only if you know what you're doing.

Want a truly easy to use, all-round audio opamp? Use the 072. It's more than quiet enough for most purposes, and will perform well even if the circuit it is used in is not designed with utmost care.

will running lower voltages affect sound quality

The supply voltage determines the maximum signal level the circuit can handle. The higher your supply voltage, the higher your maximum signal level. Otherwise, it doesn't make any difference as long as you don't drop below the minimum rated supply voltage for the particular type of opamp.

[loudthud]

An opamp spec sheet will specify an input common mode range. If you drive the input too hard, the output won't just clip, it will unpredictably slam the opposite rail. This sounds really bad. The input must be clamped so as not to allow the input to exceed the input common mode range. A low supply voltage means the input must be clamped at a lower voltage.

There was recently a thread with a schematic for an effects loop, I think it was a Marshall 1987X. It ran off of a 30V supply derived from B+ and had input clamping diodes.

Found it! Here's a link to the schematic, the effects loop is on page 4.
http://www.webphix.com/schematic%20h..._50w_1987X.pdf

[flyingdutchman]

In theory, exceeding the opamps input common mode range might get you into trouble. In practice, this very rarely happens in real-life audio. Input common mode range is only ever an issue in non-inverting circuits, and these always have a gain of at least unity. If you're driving the stage so hard as to risk exceeding input common mode range, the output will probably be well into clipping already.

That said, it's never a bad idea to apply some form of protection to inputs. Never assume no one would be so stupid as to connect a speaker jack to your line level input...

[Enzo]

If your friend loves the Delta Blues FX loop, it runs on -30v single sided, which would be the same as running on +15 and -15. So trying to run the same chip in a 6v circuit is not going to yield the same results. But that circuit doesn;t even use an op amp, it has a single emitter following transistor. So I don;t know how you can possibly compare them.

4558 op amps are as common as it gets, they were a workhorse all through the 1980s. Newer versions like the 4580 serve the same these days.

[chunkitup]

You are correct enzo. The PV delta doesn't use the opamp for fx. I have to call him to see which amp it was because I know it was a opamp fx loop. I really hope that the loop will be transparent in the sloclone I built him. Which PV amps use opamp fx loops? Loudthud thanks for that link. I will have to pull from the B+ rail.

[J M Fahey]

What +/-V rails are we talking about?
A normal center tapped filament winding (actual tap or 2 x 100 ohms resistors) has only 3.15 VAC per side, some +/-3V DC rails.
MOST regular Op Amps (TL072/RC4558/NE5532) perform very poorly at such low supplies.
Don't forget that output pins usually can't get closer than some 2 V to power rails.
They have practically no breathing space here.

One humble Op Amp I often use in such cases is LM358 .
Not a stellar performer by any measure, but it was designed to work properly from a single supply +5V rail and +/*3V is fine.

Although not the best Op Amp out there, it has saved me countless times and considering these are Lo-Fi , narrow band, distorted signals, it goes unnoticed.

[flyingdutchman]

If you have a center tapped heater secondary, you can use the circuit attached here to get a decent split supply for opamp circuitry. It uses a charge pump to obtain a raw DC supply of approx +/- 12V, with Zener diodes and series resistors to obtain a stable and sufficiently clean +/- 8V. It will easily supply 10mA, enough for most dual opamps. With lower values for the series resistors (100 or 120 ohms) 20mA should be possible.

If your heater secondary does not have a center tap but is grounded instead through 2 resistors or a hum balance pot, I would not recommend using it for powering auxiliary circuitry as this will probably result in severe hum and buzz.

[chunkitup]

That is excellent flyingdutchman! Thank you for that info.

A quick edit on my post: I do have a CT for the heater but it is elevated. Will that cause any problems?

[flyingdutchman]

A quick edit on my post: I do have a CT for the heater but it is elevated. Will that cause any problems?

If by "elevated" you mean that the CT is not connected to ground, then yes, this may cause problems. The purpose of the center tap is to make the heater voltage balanced to ground, so as to minimize hum in low level preamp stages. Also, in order for the circuit I posted to work correctly, the heater secondary CT must be grounded.

A little caveat regarding the circuit I posted: the input should be connected as close as possible to the power transformer. All rectifier circuits with capacitive loads draw current pulses at the peaks of the AC waveform, and you don't want these pulses traveling along the entire heater chain. Keep the input wires short, and twist them together neatly.

[chunkitup]

Well it is connected to ground but at about 50v to help with any hum. I am able to run my switching circuit with the elevated ground

[flyingdutchman]

Well it is connected to ground but at about 50v to help with any hum. I am able to run my switching circuit with the elevated ground

Connecting the heater supply to a DC voltage above ground as far as I know doesn't mitigate hum, the true purpose of this trick is to reduce the DC voltage differential between cathode and heater if you have some valve with its cathode potential considerably above ground. Such situations may occur in certain circuits (totem pole or SRPP) used in HiFi preamps, where the cathode of one triode is connected to the anode of another. The cathode of the "upper" triode thus sits at about half the B+ voltage.
It isn't really an issue in instrument preamps. A cathode follower may have its cathode quite a bit above ground but I've never seen any problems which would justify lifting the heater voltage.

The circuit I posted won't work with such an "elevated" heater voltage, it needs the CT solidly connected to ground.

[J M Fahey]

Well it is connected to ground but at about 50v

Then it is NOT connected to ground.

[chunkitup]

Suddenly I feel like I haven't learned anything in the 5 or 6 years since I started working with amps. I thought that it was the ground for the heater circuit. Perhaps just the return path.

Edit: Feeling blue now that cold weather has returned and stupid because my self education leaves me relying on you smart people to help me out because i cant retain anything. Good ole enzo is hurting and he chimes in to help. Thanks bro. Good people!