old Ampro 16mm valve amp projector

[paddy ryan]

Hi folks,
I am back on trying to get reasonable sound on my modified Ampro projector. As I mentioned some time ago, the amp has been stripped out on the modification so the sound is picked up by the film track passing over a photo electric cel in the sound drum , through a lead to a 6.5mm jack, which I have plugged into a pre amp, then through an amplifier, to the speaker.. so the system all works, after a fashion. Reason is I have brought a little pre amp box, and a pre owned qtx q240 amplifier
( from e bay), to get the sound through to an 8 ohm passive speaker. So now there's plenty of loud volume, but its badly distorted. I can hear the soundtrack, but its garbled with a lot of noise. I have tried changing the connecting cables from 1 to 2 ring jacks, but no change. The pre amp is a little 6 volt Cadisi job, cheap (30 quid) but with good reviews from e bay. By the way the sound drum and exciter lamp on the projector is in good nick, I have checked that out.
Any advice please?

 

[rjenkinsgb]

What is the model number of the preamp? The only ones of that make I can find are "Phono" preamps, for use with turntables?

The input circuit and loading on the photocell may be quite critical for good results.

 

[Nigel Goodwin]

I agrre, the only Cadisi preamps I can find are phone ones, completely unsuitable for using with a photocell in a projector. The result will be huge distortion (due to massive clipping) and far too much bass, due to the RIAA equalisation.

I would expect the output from a photocell to be fairly high level, and fairly high impedance - a simple one opamp preamp should be all that's required, using something like a TL081. The amp you've got is just a slave power amp, so needs a decent amount of drive.

 

[paddy ryan]

The penny has dropped! It sounds like these two little (Chinese) pre amps are completely wrong then!
I have tracked down what they are;
1. Dollatek 6J1 mini vacuum tube pre-amp audio hi fi stereo pre amplifier.

2. Turntable phono pre amp stereo RCA line level amp RIAA eq.
Is there any one opamp preamp product you could recommend for me then please! the TL081? Could I get that on Amazon? Thanks for the advice! Just realizewhat the TL081 is. Does tis mean I have to think 'circuits'. Would I have to build this in? Sorry, but am a beginner in all this!

 

[rjenkinsgb]

The TL081 is just an IC suitable to use for that purpose, though I'd have gone with a TL071; very similar..

The nearest ready built ones I can find are such as these, available on ebay and amazon:

DC9-24V AC8-16V NE5532 Audio OP AMP Mic Preamp Pre-Amplifier Board Module +Cable | eBay

NE5532 Dynamic Microphone Stereo Preamp Amplifier Board. Chip (IC): NE5532 (Low Noise). 1x NE5532 Pre-Amp Board Supplied. Twin Input Channels (Stereo). Easy Board Connections. We only accept cash on collection.

www.ebay.co.uk

They do not give any gain info though, so the levels may be too high or too low & impedance not suitable..

It's stereo, just ground the input of the channel you are not using.

 

[Nigel Goodwin]

The TL081 is just an IC suitable to use for that purpose, though I'd have gone with a TL071; very similar..

Interesting - why the 71 in particular?, I simply randomly choose the 81 over the 71.

The nearest ready built ones I can find are such as these, available on ebay and amazon:

DC9-24V AC8-16V NE5532 Audio OP AMP Mic Preamp Pre-Amplifier Board Module +Cable | eBay

NE5532 Dynamic Microphone Stereo Preamp Amplifier Board. Chip (IC): NE5532 (Low Noise). 1x NE5532 Pre-Amp Board Supplied. Twin Input Channels (Stereo). Easy Board Connections. We only accept cash on collection.

www.ebay.co.uk

They do not give any gain info though, so the levels may be too high or too low & impedance not suitable..

It's quite bizarre - it gives almost no information at all, while it would be easy to extract the information you require from the resistor values - but they aren't too clear on the picture.

 

[rjenkinsgb]

why the 71 in particular?

The TL07x ones are better for low noise / low distortion audio applications, rather than just being general purpose opamps.

From what I've read elsewhere, the 07x and 08x are from the same wafers, with the dies that test out better for audio characteristics then given the 07x numbers, while the 08x may have lower offset etc.

 

[paddy ryan]

Many thanks gentlemen, I learn a little more each time because of good folks like you. I have sent off for the op amp, looking forward to some progress on this!

 

[paddy ryan]

HI again. Well I got hold of the 5532 pre amp you recommended, and tried it out. Sorry to say there was no appreciable difference in the gain through the speaker, in fact I get the same volume with or without the pre amp. So I have the choice at the moment of the low volume or the loud volume with the horrific distortion. The other pre amps I mentioned in my previous post all give the same response, so I am flummoxed! Any suggestions where I go from here please! Thanks

 

[Nigel Goodwin]

HI again. Well I got hold of the 5532 pre amp you recommended, and tried it out. Sorry to say there was no appreciable difference in the gain through the speaker, in fact I get the same volume with or without the pre amp. So I have the choice at the moment of the low volume or the loud volume with the horrific distortion. The other pre amps I mentioned in my previous post all give the same response, so I am flummoxed! Any suggestions where I go from here please! Thanks

As I mentioned above, that preamp gives no clue as to it's specifications - neither gain, not input impedance. As it specifically mentions being a mike preamp, it's presumably fairly high gain - so may be overloading.

If you have a look here:

Project 230

ESP Audio Projects - Workbench Signal Routing Panel - select from external sources, signal out - signal in level controlled utility panel

sound-au.com

Figure 3 is the simple sort of preamp you're looking for - and it discusses how to simply change the gain.

It's just as possible to alter the gain of the preamp you have, but without a schematic or values, it's difficult to make suggestions.

Possibly the easiest solution is to add a volume control at the input of the preamp, as in figure 3 above - you can then attenuate the signal entering the preamp, to avoid overloading it.

 

[paddy ryan]

Gentlemen, with reference to previous woes on my Ampro projector sound problems, the advice on the 5532 has been successful, connecting a 5532 pre amp through to a Fender guitar amp I bought at a boot sale. Learnt a lot finally, so a belated thanks for the advice given me in Nov 2022.

 

[audioguru]

Interesting - why the 71 in particular?, I simply randomly choose the 81 over the 71.

I have used TL071, TL072 and TL074 audio opamps ever since they were invented. I wrongly thought that a TL08x opamp is a TL07x that failed audio specs but they have exactly the same specs.
I do not know why they have two part numbers for the same thing.