Looking for feedback on Transformer Saturation Box schematic

[Sam0311]

Very rough but I guessed fleshed out a bit it could look something like this? RV1 = input level, RV2 = Bias control.

 

[Sam0311]

Been doing some more reading. This is where I've got to for a Preamp circuit using the solid state load idea mention previously. Would be great to eventually add in Ian's REDDEQ and a tube make up gain circuit.

I haven't actually looked at specific transformers yet so I imagine the value could change. Was a little bit tricky to work out the value of Rk but I think somewhere around 330-680 should work.

 

[Sam0311]

Sorry for spamming, I tend to get pretty obsessive when I'm interested in something. I'm thinking a more complete circuit could look something like this.

Or I guess if I wanted the EQ to be separate I could just get rid of everything from the 'EQ In' label to R9, but would then need to make sure the amplifier had enough gain to accommodate for the loss caused by the output transformer?

 

[ruffrecords]

You do not want C3 or C4, they will defeat the negative feedback..

I am not sure of the purpose of RV2. It certainly needs to be fed by a capacitor connected to the junction of RV2 and R9. The VTB2290 is a gapped transformer. You should preferably use the ungapped version VTB2291.

There is no reason to set the ratio to be 9600:150, you might just as well make it 9600:600 and have another 6dB of gain available. Do you know what load the circuit is capable of driving?

Your balanced input attenuator has 40dB of loss. Do you need that much.

A 9045 wired 1:1 is not a good choice for a tube preamp either for microphone or line level unless you deliberately want strange frequency response.

If you are going to post a schematic it is a good idea to say what you expect the circuit to do and how well you expect it to do it if you want relevant feedback.

Cheers

Ian

 

[Sam0311]

Thanks for the input Ian, sorry my minds been on overdrive for the past couple days and I’ve just been firing out ideas, I’m also in part using this thread to document key information so it’s easier to come back to. I don’t necessarily expect people to answer everything in here but any feedback is greatly appreciated.

I’ll try and clarify some things. I know this is not how this thread started out but Thor’s comments on vacuum tubes made me want to work out how to make a vacuum tube mic preamp. As well as being able to take a mic signal up to line level I want it to be designed in such a way that you can get a nice amount of distortion from it too.

I’ve removed C3 and C4 from my most recent schematic. (Sorry I know I’ve posted a lot of them over the weekend but I’ll share it below just to show where I ended up today).

RV2 was suppose to be an output level control. I wasn’t totally sure how best to add this in so this is what I landed on. I wanted input and output level controls so I’d be able to control the amount of distortion and the overall gain of the circuit. I’m guessing the capacitor would be to block any DC leaving the output? Or to block DC to the pot?

Good shout on the output transformer. I guess if it’s driving a line input, likely the minimum load it would be driving is 10k? What’s the benefits of it being ungapped?

The idea behind the input attenuator was to step a line level signal down to around mic level so the circuit reacts in a similar way with either source. Also gives the option to hit the tube really hard if you run a line level signal without the pad.

Good to know about the 9045, in my most recent schematic I’ve gone for a 200:20k transformer to increase the voltage going into the grid. Would this be more appropriate?

I’ve changed VCC to +60V since I actually looked at a data sheet instead of using an online calculator and it looks like a 12AT7 might work how I want with a plate voltage of around 60V. I don’t fully understand the patent I’ve been reading but from what I gather if VCC is 60V my plate voltage should be about 59.4-59.6.

Hope this helps clear things up

As always, appreciate your advice Ian.

Edit: I guess it probably makes more sense to make R3 and R6 200 ohms with the new transformer.

 

[thor.zmt]

Very rough but I guessed fleshed out a bit it could look something like this? RV1 = input level, RV2 = Bias control.

View attachment 107925

Sam,
You need to learn more basic electronics. I do not now have the time to teach you.

I can do a masterclass on this kind of circuit, including showing how derive operating conditions and calculate all parts of the circuit as well as to how simulate it in a few weeks.

I am attaching a commercial but discontinued design of mine, that is a "perfected" RPA. It was actually actually was quite liked by some studio people. Especially in the final mastering step.

Several 1,000 were made and sold and it's reliable. A review that describes the function is here:

iFi iTube Review

The "FX" is a version of Phaedrus "FRANCINSTIEN" (plus some extra) and the Taddeo "Digital Antidote II" respectively.

I apologise for the way it is drawn, which does not make the circuit easily to understand, I thought I had a file from TINA, but alas no. This schematic is for PCB design and the full product.

Here is a photo of the actual PCB:



You can see it is quite SMD part heavy.

The current through the tube is equal to the bias of the 2N4403 (MMBT4403 is the SMD version), so the tube operates in "starved anode" mode (~ 24V/0.1mA). Big advantage, extremely long life and ton's of harmonics. The second "advantage" was not one for "HiFI" use and the later version dropped HD a lot and used a much higher anode voltage and current.

The Mosfet can be replaced by IRF710 or similar without other change. Design center is 3V Vth which equates to 4mA in the 2N4403. The Mosfet follower at the output runs at ~10mA so max output into 600 Ohm is only 4.2V (+14dBU)

The follow-up version made a number of other changes. The PNP Transistor was replaced by a P-Channel Mosfet, the positive supply was raised to 48V from 24V, the current source NPN was replaced with Mosfet and most crucially a DC servo was added to eliminate the output capacitor and also the capacitors in the feedback loop. It is also a lot more complex and not really suited to DIY.

Should anyone wish to produce a commercial product based on this, I suggest to contact me for the updated version and other changes and naturally a licencing agreement. I can also help apply the same principle to other products (e.g. Microphone Pre etc.).

The circuit posted here is completely depreciated and outdated and not recommend for mass production, but it is suitable for DIY assembly, once the SMD parts are replaced by TH parts.
I would still roll in some of the updates I applied to the later version and rearrange the feedback loop to include the coupling Cap and feedback DC blocking Cap.

We will do all of this in the masterclass.

Thor

 

[merlin]

I'm thinking a more complete circuit could look something like this.

There's a bunch of stuff on the input that you can get rid of

 

[ruffrecords]

This is definitely way off topic of the original thread. I think if you really want to create a tube mic pre it would be best to start a new thread. You might also like to do some research - there are lots of tube mic pre schematics here and on the web. Also check out this thread:

Feeler: Classic Solo

Cheers

Ian

 

[thor.zmt]

This is definitely way off topic of the original thread. I think if you really want to create a tube mic pre it would be best to start a new thread.

Agreed, we got way OT.

New thread for RPA Theory:

Theory, Practice and Sexlife of the "Reflected Plate Amplifier" Hybrid, low Voltage Tube circuit

Thor

 

[Sam0311]

Haha sorry guys I got a bit too excited, I’ll head to the new thread for anything further on this. Thanks for all the help so far. I’m aware I was getting ahead of myself with the tube stuff (and how off topic it was), I was just getting a buzz out of trying to work out what was happening.

Thor I want to reply to your message properly but will have to wait till after work to be able to read through it properly.

 

[Sam0311]

Sam,
You need to learn more basic electronics. I do not now have the time to teach you.

Hi Thor,

I’m very much aware that I have a lot to learn. The things is I find it very hard to learn just by reading things, it’s also not always very easy to find relevant information on the internet, even harder to find the information that’s explained in a way that makes sense to me.

Most of the time you either get given so much information it becomes a bit of a case of information overload. Or the writer either assumes a certain amount of prerequisite knowledge/it’s not practical for them to explain every last detail and you end up having to go down a rabbit hole trying to figure out what they meant by one small statement.

I think it’s so much easier to learn by asking loads of questions (and try not to worry too much that it might annoy some people, sorry ) and actually doing things. I don’t know anyone in real life who’s brains I can pick which is why I’ve resorted to drawing dodgy schematics and hoping some nice people on the internet will tell me what I’m doing wrong.

Master Class sounds very interesting, what we’re you intending? Would it be private tuition type deal? Video/article? Is it something you’d want payment for?

Need some more time to get stuck into the theoretical part of you post.

Cheers,
Sam

 

[leadbreath]

No, the best way to learn is to actually build ****.

 

[leadbreath]

Yiu have a basic circuit idea, knock it together and let us know how it sounds!!???

 

[leadbreath]

https://groupdiy.com/threads/transformer-coloring-box-again.77975/

 

[Sam0311]

No, the best way to learn is to actually build ****.

Haha good point

 

[ruffrecords]

I think it’s so much easier to learn by asking loads of questions (and try not to worry too much that it might annoy some people, sorry ) and actually doing things. I don’t know anyone in real life who’s brains I can pick which is why I’ve resorted to drawing dodgy schematics and hoping some nice people on the internet will tell me what I’m doing wrong.


Cheers,
Sam

People around here are very willing to help you build existing projects without being judgemental. Having said that, you should not expect to be spoon fed. DIY does mean acquiring certain skills and as soon as you go off piste so to speak and start wanting to develop your own designs then there are additional technical skills you need to acquire and you should make the effort to acquire them. There are ample recommended resources already posted here that you can investigate and find the one that best suits your abilities and preferred way of learning.

Cheers

Ian

 

[Sam0311]

Makes sense. I kind of naively had it in my head that for people who are well versed in electronics having a glance at a schematic and being able to give some quick pointers wouldn’t take too much work. I find it quite it quite fun trying to answer questions on music or recording if the question is something I can answer with fairly minimal effort so assumed it would be a similar deal here. It’s become quite apparent from the response here that I was asking too much of people and I do apologise for that, it’s just a bit of a misunderstanding on my part.

I guess I also didn’t fully appreciate the amount of knowledge required to design your own circuit. I’m normally able to get fairly competent at things with the right amount of work and guidance. It means I’m forever putting myself in situations where I’m completely out of my depth but I usually push past that eventually Maybe I’d be best off putting together a few kits first.

I realise that my messages over the weekend were a bit much. When I get into something I find it hard to stop going at it until I either get where I want or get bored of it and getting advice from people helps me avoid getting stuck on something. I should restrain myself from posting in future and chip away in my own time.

Sorry Ian I didn’t mean to be a pain, I think my enthusiasm just got the better of me.

 

[ruffrecords]

No Problem. You are not a pain and there is absolutely nothing wrong with enthusiasm. I think you are spot on when you said you underestimated the amount of knowledge required to design your own circuit. The advent of op amps has made audio design apparently simple but the reality is that real world engineering needs just as much skill as it ever does.

I often imagine having a conversation with Sheldon Cooper (Big Bang Theory) when he says that physicists have to understand the universe and he looks down upon engineers as mere craftsman.I would say to him that understanding the universe is the easy part. Engineers do the hard part of bending the universe to our will.

Cheers

ian

 

[Sam0311]

You make a good point. I think it’s more than just Op Amps though. Truth is I never set out to try and understand how circuits work, I just wanted some fun projects to get stuck into but the thing I enjoy most in life is the act of creating so the thought of working out how to do things myself quickly became more appealing than buying a kit.

There’s quite a lot of things in electronics that are fairly easy to get to grips with on the surface. Simple Op Amp circuits, how and why you might use a Transformer, voltage dividers, RC filters etc. As long as the concept is fairly easy to wrap your head around and there’s a simple formula you can use it’s not impossible to start chucking things together. The hard part is having a deep understanding of what’s going on and how all the different parts interact with each other.

 

[dogears]

The hard part is having a deep understanding of what’s going on and how all the different parts interact with each other.

when you get that sorted can you please let me know?