Cassette tape delay

[Nigel Goodwin]

Pretty cool but not so long delaytime as I thought, even if I run it in the slowest speed.

That was always fairly obvious, the heads are VERY close together, and the speed is 1 7/8 ips, so the delay is tiny.

 

[dr pepper]

So cassettes are 1/2 the speed of later stereo reel 2 reel.
Amazing they got as good as they did, like a lot of stuff old hat now.

 

[Rorut]

Good.
Sounds like you have some feedback, did you use screened cable?
Perhaps both issues you mentioned are due to the signal overdriving, if that is the case a resistor on the signal (non ground) end of the potmeter track might fix it.
I cant remember what speed compact cassette runs, 4 track reel to reel is something like 3 3/4" per sec, the 2 heads in a compact cassette are fairly close together so it wont take long to get from one to the other, hence a short delay, moving the heads apart wont be easy!

Yes, screened cables, except about a few cm close to the board. Did not integrate it yet inside the unit.
Do you refer to the feedback pot? That has already a 1.5K resistor in that end.
Or which one?

Thanks!

 

[Nigel Goodwin]

So cassettes are 1/2 the speed of later stereo reel 2 reel.

'Sort of'

Reel to reel were various speeds, 3 3/4 was only one (the common one for cheap domestic recorders), most were multi-speed, with 7 1/2 for HiFi quality, 3 3/4 for middling quality, 1 7/8 for poor quality, and 15/16 for talking books. Top end machines were 15 ips, or even 30 ips.

Over the years I've had to speed convert various tapes for people, a little tricky when you don't have a machine that goes fast enough

 

[dr pepper]

I used to (might still have in the garage) an ampex 4 track, it does standard speed & hi-fi speed, or whatever you callit.
I should oik it out and put it on ebay, probably fetch a packet, only thing is the belts will have turned to black sticky marks.

R11 & R17 I guess would be a start, try a 22k in series.
Try putting it in place temporarily, there will be some noisy spots & some Ok ones in the set.

 

[Rorut]

Tested with 22k but did not help the distortion on one of the end of
dry pot, a bit squeling gone and also some delay.
I also tested to connect R11 to R17 unsoldered the ground positions and connected them together, that gave interesting results and distortion gone doing that. But dont really know whats happening to the circuit connecting the pots together like that?
Mixing the wet and dry together?

 

[dr pepper]

Disconnecting the pot grounds will more or less make them act like 1 that goes from wet to dry, only you'll always have a little wet and a little dry.
If that stops the squealing maybe theres some odd phase thing happening creating oscillation conditions, are the 100p caps present & correct?

 

[Rorut]

I connected the pots to ground again.
The 100p should be correct positioned but could not measure them with my multimeter. Should I replace them?
Here is a video. The squealing can be heared about 0:40
**broken link removed**

 

[dr pepper]

Sounded good to me, just like the copykat used to.

Either my hearing is shot which is quite possible working in a noisy plant or there was no squealing on the recording.
If the 100p's are in and they are marked 100p (101) no reason to replace them.
There will be a point where gain gets high enough to amplify noise which on each echo will get louder & louder untill you get squeal, I think this is happening, if so theres not a lot you can do as its a hardware limitation, the best course would be to reduce the gain control so you cannot turn the gain up high enough for it to occur(hence my 22k idea), it makes a more professional unit where everything 'works' nicely.
Are you using a good quality tape?, naff ones hiss like a bad tempered cat, also try turning the tone down on the marantz.
Th eother thing in the back of my mind is the bias oscillator in the recorder affecting things, its a bit soon in the Am to work that one out now, I'll think about that one.

 

[Rorut]

It happens only when the feedback pot is above 70% or the rec knob on the unit is above 5. The 22K resistor on the wet pot removed much of the delay, almost all. My 100p i marked 101. I would very much like to get rid of the squealings.
For now I only have a 100 min Basf chrome tape but a friend gave me a used "BASF FERRO SUPER LH I 90", maybe that gives better results? Will try that later. I will also try the low and high pass filter later... Thanks!!

(Bias oscillator sounds interesting )

 

[dr pepper]

Ok do that.
Tape recorders, apart from really nasty ones have a bias oscillator, the magnetic material in the tape doesnt respond that well to high frequencies when recording so they put a continuous tone of the tape high up in the audio range to kinda excite the tape and improve high end response. If this was to end up being amplified & re-recorded that might make the squeal.
I dont know if the likes of the copykat had a bias system or not.
Increasing the size of the feedback cap on one of the amps would decrease the likelyhood of this happening, however that would remove some treble and mess up sound quality.
My description of tape bias system is probably no where near audiophillic, not was it meant to be, I'm not interested in that kinda thing.

 

[Nigel Goodwin]

I dont know if the likes of the copykat had a bias system or not.

They did (even the valve ones), tapes are heavily distorted without bias.

 

[Rorut]

Ok do that.
Tape recorders, apart from really nasty ones have a bias oscillator, the magnetic material in the tape doesnt respond that well to high frequencies when recording so they put a continuous tone of the tape high up in the audio range to kinda excite the tape and improve high end response. If this was to end up being amplified & re-recorded that might make the squeal.
I dont know if the likes of the copykat had a bias system or not.
Increasing the size of the feedback cap on one of the amps would decrease the likelyhood of this happening, however that would remove some treble and mess up sound quality.
My description of tape bias system is probably no where near audiophillic, not was it meant to be, I'm not interested in that kinda thing.

Thanks, dont want to mess up the quality , guess I have to live with the sounds

 

[dr pepper]

Well if its just a case of getting the settings in the right place to work Ok thats not to be unexpected on a project like this, sure a copykat commercial tape echo would work well, and possibly (not definitely) would not do screwy things no matter where you set the dials, however that will have had at least a couple of experienced engineers spend a load of time & money on it getting it right.
You stuck with it and did well.

Didnt know that Nige, so why do tapes distort without bias then?

 

[Rorut]

Well if its just a case of getting the settings in the right place to work Ok thats not to be unexpected on a project like this, sure a copykat commercial tape echo would work well, and possibly (not definitely) would not do screwy things no matter where you set the dials, however that will have had at least a couple of experienced engineers spend a load of time & money on it getting it right.
You stuck with it and did well.

Didnt know that Nige, so why do tapes distort without bias then?

Yes Im really happy with it and the problem only exist on when pushing the limits and its ok.
Really grateful for all help you gave me on this project.
But I really dont know what the benefit of the dry pot will be for me. Maybe a purpose will appear for me in the future .

I testet with the LPF and HPF filters and pot between them. It works but sounds a bit strange (not bad) in the LPF end. Think I sould like to have a switch that I can turn the filters on and off but I beleive I have to move that 10K resistor in series with the volume pot. Dont want that there if the filters is in off position. Would it work just to move it after the volume pot instead?

 

[dr pepper]

If you want the filters maybe you could put a volume pot before the filter, that way you could trim the apparent volume so that theres no change when you switch the filter in & out, that would enable you to use the bypass switch as an effect, or instead of a pot just a pcb trimmer and trim out the volume difference. You could then possibly do away with the balance pot idea between the filters.
Really glad to have been assistance.

 

[Nigel Goodwin]

Didnt know that Nige, so why do tapes distort without bias then?

It's down to hysteresis, essentially you apply 60-100V of high frequency bias, and a few 100mV of signal so as to record on a linear part of the hysteresis curve.

 

[dr pepper]

Yep makes sense I kinda knew that, what I didnt know is that without bias there would be a load of distortion, I was expecting just an early roll off in frequency.
So once you get the magnetic domains all nice & jiggly they are more amenable to taking on higher frequency weak fluctuating fields.
So 100v of bias explains why you see fairly large toko transformers in better end recorders, I wondered why they didnt just use a simple oscillator.

 

[Nigel Goodwin]

So 100v of bias explains why you see fairly large toko transformers in better end recorders, I wondered why they didnt just use a simple oscillator.

They do, it needs to be a sinewave oscillator though, and you need the transformer to generate the high voltage required. They need to be fairly powerful, as the erase head is fed directly from the transformer, and the bias is fed to the record head via a highish value resistor.

 

[audioguru]

Since the tape speed in a compact cassette is slow, it rolls off high audio frequencies so a lot of boost to high audio frequencies is done when recording.