UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 9th Jan 2020, 9:49 am   #21
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
Default Re: OP amp oscillating/modulating OP.

One opamp circuit to be careful about is an active filter. It's easy to assume the opamp only needs to be quick enough to pass the highest frequency signal the filter is to let through. But actually these circuits rely on opamp operation to produce the transition to the stop band, and sometimes the stopband itself. So you can need an opamp good for all the frequencies which can appear at the filter's input.

Cunning design of active filters is to have the design include a passive stage before the active stuff. This limits the babdwidth hitting the opamp, so you don't need such a flashy opamp. This pays for itself when say an audio signal appears amongst RF, or on the end of a delta-sigma DAC.

That subwoofer circuit includes active filter stages.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 9th Jan 2020, 10:56 am   #22
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
Default Re: OP amp oscillating/modulating OP.

The other thing to bear in mind is internal signal levels. There are active filter circuits in which the voltage swing of the op amp is significantly higher than the input voltage, or the final output voltage. Spice modelling and/or distortion vs input level is the only way to understand this.
Craig Sawyers is offline  
Old 9th Jan 2020, 11:45 am   #23
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
Default Re: OP amp oscillating/modulating OP.

Although the active filter stages in the OP's unit are straightforward Sallen & Key topology, and don't suffer from the behaviour I mentioned above.

Craig
Craig Sawyers is offline  
Old 10th Jan 2020, 7:17 am   #24
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,637
Default Re: OP amp oscillating/modulating OP.

"In fact one really good way to turn a square wave into a triangle or trapezoid, is to simply pass it through a low slew rate OP amp. It works just as well as a current source charging a capacitor and generates a beautifully linear ramp. " Interesting, so this would make a good timebase generator or part of a curve tracer step circuit?

Fitted the 5532 into the active woofer, all's well. Also tried one in my LPF, which is a Butterworth I think, it worked better than the TL072, no ringing but I also put a 10n cap between the rails as suggested.

I'll bear in mind some of your suggestions and tips in the future as I want to build another LPF and active Xover. One issue I ran into was phase shift. I wondered why the sub & stereo speakers sounded odd, it felt light your head was disjointed, all without drugs, this must have been the phase difference. I was a very odd sensation.

Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline  
Old 10th Jan 2020, 9:53 am   #25
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
Default Re: OP amp oscillating/modulating OP.

The 5534/5532 are very, very, good devices for audio and they've been on the go for 40 years. There are newer devices which beat them on paper, but the differences aren't perceptible in use. Their noise behaviour is good enough to make an excellent moving magnet cartridge preamp. Their output stages can drive 600 Ohm lines quite happily, and they're pretty tame to use. One of the all-time greats.

Don't use them for precision DC or high impedance work. Their input bias current is high.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 19th Jan 2020, 1:07 am   #26
knobtwiddler
Octode
 
Join Date: Mar 2016
Location: London, UK.
Posts: 1,046
Default Re: OP amp oscillating/modulating OP.

My understanding is that, as of Jan 2020, as a monolithic amp (not discrete, where you can get about another dB SNR if you go the bother), the 5534 is still unbeatable for noise as a MM preamp. It's years since I looked, but I believe the 34 gets its noise advantage over the 32 due to have twice as many paralleled BJTs (amongst other advantages). If there is a quieter OA for MM carts I'd love to know about it. Self's chart from 2007 is still true to my knowledge.
knobtwiddler is offline  
Old 19th Jan 2020, 10:52 am   #27
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
Default Re: OP amp oscillating/modulating OP.

Agreed. The main difference between the 5532 and 5534 relates to gain stability. The 5532 is unity gain stable, whereas the 5534 is only stable for gains greater than three (there is a compensation pin to add a small capacitor for lower gains than three).

The win is in noise. the 5532 is 5nV/rootHz and the 5534 is 3.5nV/rootHz (3dB lower).

And yes - for a magnetic cartridge there is very little to be gained by using anything with higher performance than the 5534.

Alas in common with the general trend, most of the package options are surface mount. DIP versions are still made, but it is only a matter of time that they are obsoleted.

Craig
Craig Sawyers is offline  
Old 19th Jan 2020, 12:55 pm   #28
Ted Kendall
Dekatron
 
Join Date: Jul 2008
Location: Kington, Herefordshire, UK.
Posts: 3,658
Default Re: OP amp oscillating/modulating OP.

Quote:
Originally Posted by knobtwiddler View Post
My understanding is that, as of Jan 2020, as a monolithic amp (not discrete, where you can get about another dB SNR if you go the bother), the 5534 is still unbeatable for noise as a MM preamp. It's years since I looked, but I believe the 34 gets its noise advantage over the 32 due to have twice as many paralleled BJTs (amongst other advantages). If there is a quieter OA for MM carts I'd love to know about it. Self's chart from 2007 is still true to my knowledge.
In the second edition of Small Signal Audio Design, Self examines the LM4562, which beats the 5532/4 on some parameters (not by much, consudering the 30-year design gap), but concludes that the 5534 is still better for MM pickups on account of its lower current noise. Remarkable chip, indeed.
Ted Kendall is online now  
Old 19th Jan 2020, 7:28 pm   #29
knobtwiddler
Octode
 
Join Date: Mar 2016
Location: London, UK.
Posts: 1,046
Default Re: OP amp oscillating/modulating OP.

I cannot think of an OA less suited to MM cart impedance than a 4562. The really odd thing is that NSC put a phono amp in the apps data! Aside from being many dB noisier than the humble 5534, such a source Z impacts the 4562's stability. For low-Z sources the 4562 is fantastic - and a bargain. Get them while you can.
knobtwiddler is offline  
Old 19th Jan 2020, 8:53 pm   #30
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
Default Re: OP amp oscillating/modulating OP.

It's always amusing to read the beginning of a datasheet. The marketing departments seem to just list all the currently trending products with electronics in them, without any consideration for the suitability of the part. Absolutely everything now has to be claimed to be the bee's knees for the internet of things (groan!), for plug-in hybrid cars, tablet computers and drones.

For a given semiconductor, whether discrete or integrated with others on a chip, there is an optimum source impedance for the lowest noise contribution. For RF applications, the optimum impedance isn't necessarily purely resistive, there often is a reactive component (inductive or capacitive) as well. The fates decree that the optimum source Z for noise isn't the same as the optimum for gain. As far as a record playing cartridge amplifier is concerned, everyone is aware that cartridge makers specified a resistive load impedance to be applied to the cartridge to tame its resonances and give a flat response (after RIAA) usually 47k Ohms

This is not the same as saying that the cartridge looks like a 47k resistive source to the amplifier, they are a lot lower than that and a good preamplifier is optimised for the actual source impedance the cartridge presents.

Moving coil cartridges are even lower impedance and give even lower output voltages. 'wide' transistors or arrays of parallel ones are particularly suited to low source impedances and so these are the tools of choice for MC cartridges.

The NE5534 is close to ideal for MM cartridges. It gives low noise, and its lowest noise close to the impedance a MM cartridge presents.

Whenever choosing a low noise amplifier or transistor, don't take the first figure they trumpet. You need to know the noise figure at the frequency and with the source impedance you intend to use it with. These factors make big differences.

The NE5534 just happened to be very close to optimum for MM cartridges. A little progress has been made since, and the more targeted processing of discretes can do a little better still. It's an accolade that these improvements are as little as they are.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 1:26 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.