Narrowcast V2  [documentation]

Original effects with schematics, layouts and instructions, freely contributed by members or found in publications. Cannot be used for commercial purposes without the consent of the owners of the copyright.
Post Reply
User avatar
dylan159
Resistor Ronker
Information
Posts: 267
Joined: 18 Jul 2018, 14:02
my favorite amplifier: Chempion!
Completed builds: Oh boy...
Has thanked: 97 times
Been thanked: 534 times
Contact:

Post by dylan159 »

It's been a year and a half since I made the Dudson Narrowcast, a reworked version of the popular Hudson Broadcast. It was fun and sounded nice, but it wasn't perfect, and I left it sitting there. The issue I couldn't gulp down was that i had to omit the feedback lowpass capacitor because of oscillation issues. That and more has been fixed in this V2. This update is also an intriguing perspective in how much I learned since then. There's always more, but at least I feel like I understand the circuit enough to know what to do.
narrowcast2.PNG
The changes are pretty substantial so that I didn't list them one by one anymore, but in theory the sound should be the same. The first important thing is that as the original, it's a series-shunt feedback amplifier, with AC only global feedback. This is what I rebuilt this around, and as you notice the first stage is greatly simplified. Gone is the bypassed 5.6K emitter resistor, since it doesn't affect AC operation. R5 stays as is, and it's bypassed by C6 in variable amounts depending on the gain mode switch (which also means different amounts of feedback). R9 serves pretty much the same function as the 56 ohm in the original, but it's now in series just with the bypass capacitor because it seems more elegant (makes no difference for DC bias since it's in series with 1K). The base bias is simplified too. Instead of taking DC feedback on a tap on the collector, I've taken it directly at the collector. R3 and R2 have been chosen to give the desired bias, with R3 being large enough to not cause loading, and R2 to keep the original input impedance. This feedback is DC only, so it doesn't affect gain or input impedance. Since the base voltage needs to be low, the two resistors are enough to drop it.
I've removed some degeneration on the emitter, but I actually have more DC feedback from the collector. How does this affect bias stability? It actually doesn't that much! Probably the coolest characteristic of the original is there pretty much intact: bias is almost impervious to supply voltage changes or beta variations, which is why the original can run at 27V too. In simulation, the collector voltages were pretty much multiplied by the same amount as the supply is, with Q2 being 19.5V at 27V supply (6,5*3=19.5! :) ). In practice this means that you can try any voltage up to 27V and any combination of transistors and you can expect the circuit to behave as intended with no changes.
Q2 is pretty much the same as the original, with the obvious exception of being NPN. I'm not preventing anyone to try this with PNP transistors, so for that there's the version below. The Q1 has been rebiased to have about 6,5V on Q2's collector (why not 4,5V? the original biases at about 2.5V, but being NPN instead, the same bias should be taken from the positive rail, so that gives 6.5 the off-center bias is an important part of the sound so I kept to it), and simulation shows the headroom is the same as the complementary version, so I guess I've done things right.
Some things are gone, some things are back. Like the power supply filter resistor, or the gain stopper that prevents the output from cutting off, or C11. Oh how glad I am to get C11 back. I never had luck and always got oscillation with it, no matter if I was breadboarding the narrow, the broadcast, with silicon or germanium, with or without transformer. In the end some general bandwidth limiting/dominant pole compensation did the trick with C5, which doesn't otherwise affect response much at all. I guess you really need poor bandwidth to begin with otherwise, and even my germs were too good...
Did I say transformer? That one is on the bench still. I even got a proper "10K" transformer and made comparison, and I didn't hear anything so I keep my stance. And since it makes the circuit more affordable and easy to build, I'm all for it. A 25K or 50K volume pot gives a similar load as the original, accounting for all the series and parallel output resistors. Taper is log.
As it's usually the case for me, values have been standardized to the E6 series where this doesn't mean a compromise, with gain and AC response (in this case I've also done transient analysis) as close as the original as possible. Here's a sweep of gain and bass controls comparing narrowcast and broadcast (minimum and maximum bass, four gain settings) in low gain mode:
lowgain.png
and high gain:
higain.png
Here's the PNP version. The change in the Q1 base bias gets reflected in the rest of the circuit and biases Q2 to the desired 2.5V with no other changes:
narrowcast2pnp.PNG
Here's a compact and verified vero layout:
narrowcast2.png
And finally demo:
Personal blog/archive: https://bentfishbowl.wixsite.com/electronics
Find me at https://discord.gg/bMuhX4TkZM Audio Electronics discord server.

User avatar
MGaburriJazzDude
Information
Posts: 12
Joined: 23 Feb 2015, 20:25
Location: Buenos Aires, Argentina.
Has thanked: 6 times
Been thanked: 4 times

Post by MGaburriJazzDude »

Wow, I not only admire your results, but also your motivations.
It's full of electronic MONSTERS around here, but I find your submissions the most interesting, by far!

Thanks!

User avatar
dylan159
Resistor Ronker
Information
Posts: 267
Joined: 18 Jul 2018, 14:02
my favorite amplifier: Chempion!
Completed builds: Oh boy...
Has thanked: 97 times
Been thanked: 534 times
Contact:

Post by dylan159 »

MGaburriJazzDude wrote: 01 Jul 2021, 00:13 Wow, I not only admire your results, but also your motivations.
It's full of electronic MONSTERS around here, but I find your submissions the most interesting, by far!

Thanks!
You make me happy. this is all I need, really.
Personal blog/archive: https://bentfishbowl.wixsite.com/electronics
Find me at https://discord.gg/bMuhX4TkZM Audio Electronics discord server.

User avatar
287m
Breadboard Brother
Information
Posts: 143
Joined: 11 Nov 2014, 22:51
Location: Heaven ~ Hell
Has thanked: 481 times
Been thanked: 74 times

Post by 287m »

Sound great for my ear.

Oh NO! You make me feel easier to killing time.
had fun make layout for this, of course with help of coffee and cigar

Thank you!

User avatar
dylan159
Resistor Ronker
Information
Posts: 267
Joined: 18 Jul 2018, 14:02
my favorite amplifier: Chempion!
Completed builds: Oh boy...
Has thanked: 97 times
Been thanked: 534 times
Contact:

Post by dylan159 »

287m wrote: 01 Jul 2021, 14:18 Sound great for my ear.

Oh NO! You make me feel easier to killing time.
had fun make layout for this, of course with help of coffee and cigar

Thank you!
PCB layout i guess! same as last time, have fun indeed.
thanks by the way.
Personal blog/archive: https://bentfishbowl.wixsite.com/electronics
Find me at https://discord.gg/bMuhX4TkZM Audio Electronics discord server.

User avatar
MGaburriJazzDude
Information
Posts: 12
Joined: 23 Feb 2015, 20:25
Location: Buenos Aires, Argentina.
Has thanked: 6 times
Been thanked: 4 times

Post by MGaburriJazzDude »

dylan159 wrote: 01 Jul 2021, 08:15
MGaburriJazzDude wrote: 01 Jul 2021, 00:13 Wow, I not only admire your results, but also your motivations.
It's full of electronic MONSTERS around here, but I find your submissions the most interesting, by far!

Thanks!
You make me happy. this is all I need, really.
:3 Catch you on that discord server!

User avatar
alex.spaceman
Information
Posts: 5
Joined: 12 Jan 2012, 18:08

Post by alex.spaceman »

Bit late to the party but this looks brilliant! Been looking for a transofrmerless Broadcast layout and this is kuch more that just that. Will be keen to give it a spin. Huge kudos @dylan159 👍

Couple of simple questions... Were I to try and slip in one of my Soviet NON Ge trannies in Q2, would the bias need tweaking at all? And looking at the two schems, it seems to me that wiring for a Dual version would be identical to the original Broadcast taking the switch to a 3pdt along with the the two volume knobs - or am I missing something?

User avatar
dylan159
Resistor Ronker
Information
Posts: 267
Joined: 18 Jul 2018, 14:02
my favorite amplifier: Chempion!
Completed builds: Oh boy...
Has thanked: 97 times
Been thanked: 534 times
Contact:

Post by dylan159 »

alex.spaceman wrote: 24 Aug 2021, 09:51 Bit late to the party but this looks brilliant! Been looking for a transofrmerless Broadcast layout and this is kuch more that just that. Will be keen to give it a spin. Huge kudos @dylan159 👍

Couple of simple questions... Were I to try and slip in one of my Soviet NON Ge trannies in Q2, would the bias need tweaking at all? And looking at the two schems, it seems to me that wiring for a Dual version would be identical to the original Broadcast taking the switch to a 3pdt along with the the two volume knobs - or am I missing something?
As you said, much more and still the same! if you wanted to omit the transformer you could just have done that.
The bias is very stable thanks to both emitter degeneration and dc feedback (for Q1), so you can use about anything. use either the NPN or the complementary version depending on the case.
I was looking at the dual with a guy on reddit, you can do the same thing but I suggest making the 470R and 47R your trimmable gain presets, or even just the 470, since gain is a ratio of the pot to that roughly, and for all other purposes (input impedance) that's why you have the two modes. Unlike in the dual, doing it this way doesn't mess with your bias at all. alternatively you can use two separate gain pots, probably in series and shorting either one out, so there's no open loop. That will give you two completely independent gain settings. You can choose to keep the gain toggle still.
Personal blog/archive: https://bentfishbowl.wixsite.com/electronics
Find me at https://discord.gg/bMuhX4TkZM Audio Electronics discord server.

User avatar
szukalski
Information
Posts: 10
Joined: 30 Nov 2022, 13:01
Has thanked: 36 times
Been thanked: 14 times

Post by szukalski »

I built this from the blog, which in turn sent me here. Really appreciate the great work and sharing the love. I had a Duocast with transformer, and this audibly equivalent and practically better!

User avatar
dylan159
Resistor Ronker
Information
Posts: 267
Joined: 18 Jul 2018, 14:02
my favorite amplifier: Chempion!
Completed builds: Oh boy...
Has thanked: 97 times
Been thanked: 534 times
Contact:

Post by dylan159 »

szukalski wrote: 13 Mar 2023, 11:33 I built this from the blog, which in turn sent me here. Really appreciate the great work and sharing the love. I had a Duocast with transformer, and this audibly equivalent and practically better!
Thanks. Nice to see the blog is getting some visibility too and there's traffic going both ways. Newer posts will also link to FSB.
"audibly equivalent and practically better" haha I like that :lol:
Personal blog/archive: https://bentfishbowl.wixsite.com/electronics
Find me at https://discord.gg/bMuhX4TkZM Audio Electronics discord server.

User avatar
ozmuth
Information
Posts: 1
Joined: 04 Oct 2021, 05:35
Has thanked: 1 time

Post by ozmuth »

Hi there, I generally love the sound of this circuit - out of curiousity I hooked it up to an oscilloscope and noticed that the signal is not clipping anywhere near symetrically. See attached.

I have tried both NPN and PNP version and I had even layed out a PCB of the circuit and also breadboarded and all have this asymterical clipping. Im not sure if the original broadcast circuit has this. I love a bit of asymetrical clipping as much as the next person but this seems a little too severe to be desirable.

Any ideas what could be causing it?
Attachments
hi-gain-clipping.jpg
hi-gain-clipping.jpg (10.63 KiB) Viewed 6742 times
low-gain-clipping.jpg

User avatar
dylan159
Resistor Ronker
Information
Posts: 267
Joined: 18 Jul 2018, 14:02
my favorite amplifier: Chempion!
Completed builds: Oh boy...
Has thanked: 97 times
Been thanked: 534 times
Contact:

Post by dylan159 »

ozmuth wrote: 13 Dec 2023, 02:08 Hi there, I generally love the sound of this circuit - out of curiousity I hooked it up to an oscilloscope and noticed that the signal is not clipping anywhere near symetrically. See attached.

I have tried both NPN and PNP version and I had even layed out a PCB of the circuit and also breadboarded and all have this asymterical clipping. Im not sure if the original broadcast circuit has this. I love a bit of asymetrical clipping as much as the next person but this seems a little too severe to be desirable.

Any ideas what could be causing it?
I think that's expected. The second stage follows the same bias and has the same degeneration as the other one. Degeneration has the effect of reducing output headroom asymmetrically, but the amount that it does is preserved, according to the first post. It's not easy to maximize symmetrical output with degeneration, but it was never the goal here!
Personal blog/archive: https://bentfishbowl.wixsite.com/electronics
Find me at https://discord.gg/bMuhX4TkZM Audio Electronics discord server.

Post Reply