Finally stepping out of my comfort zone

Govmnt_Lacky · February 17, 2013, 07:03:29 PM

So after being here for a while

I have decided to try designing a delay circuit from some of the Reticon R510x chips that I have. I am using the datasheet circuit as a starting point however, I have neither the design experience nor the complete understanding of different audio "stages" to move forward.

The datasheet I am referring to is here:

http://www.experimentalistsanonymous.com/diy/Datasheets/RD5108.pdf

Specifically Sheet 4. As you can see, the input is shown DIRECTLY into the BBDs input via an electro and being biased with the circuit's Vdd voltage. So my question is...

Doesn't there need to be some form of input buffering or an input gain stage via an op amp or transistor network? Any other delay based circuit I have worked on has NEVER gone directly into the BBD. Could someone perhaps point me to some literature that will help me understand this particular project?

From what I see, it appears that Q2 and the 741 are in place to create some form of recovery stage but, I am probably talking out of my arse as I have no clue

Please help me understand what is going on here. Thanks!

gcme93 · February 17, 2013, 07:34:00 PM

Go for it!

This may be a silly suggestion from a novice, but maybe just breadboard it and see? Play around with it. If you think it needs more oomph in front, copy an input stage from something simple, play around with the gain in front of it?

It would be awesome if you kept a design log/diary kinda thing to share afterwards (even if it was very rough)

George

Govmnt_Lacky · February 17, 2013, 07:46:03 PM

Thanks George!

I realize this is an extinct chip BUT, I have a few and would like to use them for SOMETHING!!

This is new for me as I have not gotten into anything this deep on a "design" level. I am usually just following directions and/or making tiny, tiny leaps into the unknown.

I am hoping to get some form of input however, from the research I have done, it looks like there are not many that like to deal with BBD based circuits.

Not looking for a handout persay... Just looking for some direction

Gurner · February 17, 2013, 07:50:30 PM

The datasheet indicates a typical input of 1.5V RMS....therefore yes, unless you want your raw guitar signal to be lost in the IC's noise floor, then you'll need a gain stage in front.

The input 'shunt resistance' is 300k (which if I'm interpreting that correct is probably the IC's input impedance....not outrageously low, but a gain stage would eradicate any worries on that aspect.)

So, I'd say look at putting an an adjustable gain stage in front of the IC ....1,5V RMS is about 4V peak to peak....a typical guitar signal initial transient is about 1V peak to peak (though this can vary with pickups & playing style), so I'd say a gain stage adjustable between 3x and 8x will get you in the ballpark.

Depending on your follow on circuity you might want to take some of the magnitude out of the output signal too (a simple output pot) as 1.5V RMS may well clip any follow on stages.

Govmnt_Lacky · February 17, 2013, 07:53:33 PM

Quote from: Gurner on February 17, 2013, 07:50:30 PM
The datasheet indicates a typical input of 1.5V RMS....therefore yes, unless you want your guitar signal to be lost in the IC's noise floor, then you'll need a gain stage.

The input 'shunt resistance' is 300k (which if I'm interpreting that correct is probably the IC's input impedance....not outrageously low, but a gain stage would eradicate any worries on that aspect.

SO I'd say look at putting an an adjustable gain stage in frontvof the IC ....1,5V RMS is about 4V peak to peak....a typical guitar signal initial transient is about 1V peak to peak (though this can vary with pickups & playing style), so I'd say a gain stage adjustable between 3x and 8x will get you in the ballpark.

I appreciate your input Gurner... Thanks

Unfortunately, I have no idea how to construct or incorporate said gain stage

Could you point me in the right direction?

Gurner · February 17, 2013, 08:01:02 PM

It's a little small, but this is the kind of circuit you'd want....

http://i.cmpnet.com/rfdesignline/2007/06/TI_pt1_Fig2.gif

google non inverting opamp stage .....heaps of explanations better than I could muster!

Govmnt_Lacky · February 17, 2013, 08:18:20 PM

Thanks again Gurner!

I see lots of examples for a non-inverting gain stage BUT, they are all simple compared to the example that you show above. Just the op amp and 2 resistors controlling the gain amount. Your example shows several resistors and some caps which I know what a few will do but, mostly I have no idea

I feel such the NOOB!! $:-\$

armdnrdy · February 17, 2013, 09:12:49 PM

After breifly looking at the data sheet and seeing that the 5108 is a positive voltage BBD with voltage ratings of 7 to 13 volts, I don't see why this delay IC can't be retrofitted into a MN3208 or MN3205 circuit.

You can at least use those circuits as a guide to work up your own build.

The circuit on page 4 is an evaluation circuit not a production delay unit.

Govmnt_Lacky · February 17, 2013, 09:27:05 PM

Quote from: armdnrdy on February 17, 2013, 09:12:49 PM
The circuit on page 4 is an evaluation circuit not a production delay unit.

I understand this. I was merely using it as a place to start. Although, I do not see why it could not be implemented. Unless I am missing something $:-\$

Also, this is only a 1024-stage delay. The MN3208 and MN3205 are 2048 and 4096 respectively. I am looking at this as something new, not something I want to re-invent. Want to learn some stuff along the way as well

armdnrdy · February 17, 2013, 10:15:42 PM

Gotcha!

Your first post threw me when you stated that you wanted to design a "delay circuit" with this IC.

Well then I would look to some MN3207 circuits for guidance if you don't know where to start. That's what Scott Swartz did with the AD-3208. He pulled a little from here and a little from there.

Govmnt_Lacky · February 17, 2013, 10:44:50 PM

@Larry

Ultimately, I would like to design a delay circuit based on this chip. I would just like it to be unique and I would like to go through the process as I have not done that before from start to finish. I figured the evaluation circuit was a good place to start and I could build off of it. There are just a lot of things I do not understand and need to go through the process for the knowledge.

Might stop by here constantly for a point in the right direction though

Ronan · February 18, 2013, 04:42:20 AM

This is the best I have read on op-amps and using them to amplify a signal, or buffer, mix, filter:

http://sound.westhost.com/dwopa.htm

If you really need to cut to the chase, scroll down to inverting opamps and noninverting op-amps. However, if you can wrap your head around the third heading "Basic Rules of Opamps" you will never regret it!

gcme93 · February 18, 2013, 05:52:47 AM

My brief description of what the parts in this circuit do (Left to right)

Rt is an input resistor (so good nice and high)

1. First cap is an input cap. This has to do a few things:
- Allow all the frequencies that we need through (audible guitar frequencies)
- Stop any DC voltage that might be coming into the circuit from the input
- Contain the DC voltage that we're adding at the next stage which is...

2. The next unlabelled resistors from V+ and from ground set the bias point by way of a voltage divider. Use the voltage divider equation to work out what the added DC voltage is to the signal path by Bias voltage = V+ * {Rbottom / (Rtop + Rbottom)} This is the bias voltage needed by the particular op amp you use

3. The next two resistors are what define the gain of the op amp:

4. This whole section of the circuit is only dealing with the small AC signal, and we contain this part of the circuit with the cap at the bottom and the output cap (only the output cap needs to be selected to allow guitar frequencies through). Therefore it's all effectively working with the signal voltage but shifted up by the DC voltage set by stage 2

5. The final resistor is the output resistor (good low!) and there is an example of a load on the far right.

Hope this helps. Even with knowing the function of each part, it's probably easiest to mirror another circuit to save time on calculating each part. Then you know how to play with individual areas to improve the performance if you know what each part is meant to do.

Good luck!

George
(I hope my electronics modules are serving me well here, please correct anyone who spots inaccuracies)

Govmnt_Lacky · February 18, 2013, 08:31:15 AM

Thanks Ian and George!!!

Great stuff and great explanations. Gonna soak this in and try experimenting!

bluebunny · February 18, 2013, 09:56:33 AM

Sounds like a fun project to get into. Ian's link is very good - read that one before. Good ol' Wikipedia has a good introduction to opamp circuits (here) and some useful links at the bottom of the page ("Op Amps for Everyone" is a perennial favourite). I guess you're going to need buffers and mixers in there, and opamps are a neat and easy way to do these. Or some simple one-transistor circuits (e.g. at AMZ).

Looking forward to seeing how this pans out. Good luck!

Gurner · February 18, 2013, 11:00:43 AM

Just to add to Geroge's description, the two final resistors (far right of schem) will not be needed in your situation (they assume you don't know what follows on...but in your situation you do & they won't be needed)

Additionally, if you set the opamp input bias point to about half the supply voltage (those two resistors attached to the +ve input ....use identical values ...about 1M or higher), then the output cap won't be needed either...as the DC level on the ouput of the opamp will be half the supply voltage....which is what your IC will require (ish)

Govmnt_Lacky · February 18, 2013, 11:07:32 AM

Right now, I am thinking about setting the Vbias resistors at 1M each. Also, I am going to start with an R2 value of 100K and install a 1M trimmer for R1 (initially set at about 300K) and tune until I get a good input to the BBD.

Can I use the 1uF electro as the op amp's output cap? (The one that is on the input to the BBD)

Also, is there going to be a problem with the input gain stage bias VS. the BBD bias? I can assume that running at 12V, the input gain bias would be ~6V. I am wondering if that will be a problem for the BBD input bias point

Gurner · February 18, 2013, 12:43:45 PM

Quote from: Govmnt_Lacky on February 18, 2013, 11:07:32 AM
Right now, I am thinking about setting the Vbias resistors at 1M each. Also, I am going to start with an R2 value of 100K and install a 1M trimmer for R1 (initially set at about 300K) and tune until I get a good input to the BBD.

Can I use the 1uF electro as the op amp's output cap? (The one that is on the input to the BBD)

Also, is there going to be a problem with the input gain stage bias VS. the BBD bias? I can assume that running at 12V, the input gain bias would be ~6V. I am wondering if that will be a problem for the BBD input bias point

As I said in my post immediately above yours, if you arrange the DC bias for opamp +ve input pin right, then you also satisfy the bias required for the follow on IC... your IC's datasheet says the input bias should 0.6 VDD (Table 2 ...'input signal bias'), therefore if you arrange the opamp bias resistors with a 40/60 ratio, then that'll do nicely (eg top resistor 800k, bottom resistor 1.2M, then your DC level on the output of your opamp will be the ideal bias for your follow on IC & you won't need an electro output DC blocking cap.

Govmnt_Lacky · February 18, 2013, 01:00:08 PM

@Gurner

Ok. Just to clarify...

If I set the resistor between V+ and the op amps non inverting input at ~800K and set The resistor between the op amps non inverting input and GND at ~1.2M then I should have the input gain stage biased just about right to send the signal directly from the input op amp's output directly to the BBDs input without the need for the 1uF DC blocking cap?

You also mentioned above that I should aim for a 4X to 8X output from the op amp gain stage. Does that still hold water with this basing arrangement? I would think so

Cheers and thanks for helping me understand this!!!

gcme93 · February 18, 2013, 01:18:05 PM

Yepp that will put your bias at around 7.2V so that 1uF blocking cap isn't needed.

Then I'd use a variable resistor for R1 and a set resistor R2 which is about a tenth of R1's max value. This will let you play with the voltage gain from unity gain to about 10X gain.

(R1 and R2 are based on the second diagram in my last post, the labels are too small for me to read in the first diagram)

This gain amplifies the varying voltage (i.e our signal) but the signal stays shifted up at 7.2V above ground. Roughly put, its like a sinewave with an average voltage of 7.2V not 0V. It doesn't get shifted up or down on the graph, just gets a bigger sine wave amplitude.

Have fun!

George

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Finally stepping out of my comfort zone