peterc
Well-known member
Thx Pete, seemed to have missed that......
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hobiesound
Well-known member
ok people i'm nearing completion of my pcb for the am9 pre as per the simplified schematic posted earlier by Peter P.
i chose to go for oep a262a3e wired 1:3.2 for input and a carnhill vt1148 wired 1:1.5 offboard for output. Sowter was too expensive right now and i can get both of them relatively cheap in holland (hurray for the good exchange rate between pounds and euro's!! ;D ) . I also added a load resistor and a zobel network on my schematic since i'm not sure if i would need it and than i could add it later if the need would arise.
I also replaced tr9 and tr10 with bc141's so it could drive the transformer more comfortably. I'm not sure if i have to change any values surrounding tr9 and tr10 to accomodate this transistor swap. but i'm pretty sure i could just swap them without any change. right? <--- please comment!!!
I'll probably post the layout/schematic tonight since i'm busy doing some mixing right now. i would love to hear any comments on the layout since this will be my first mic pre layout.
greetings,
Thomas
i chose to go for oep a262a3e wired 1:3.2 for input and a carnhill vt1148 wired 1:1.5 offboard for output. Sowter was too expensive right now and i can get both of them relatively cheap in holland (hurray for the good exchange rate between pounds and euro's!! ;D ) . I also added a load resistor and a zobel network on my schematic since i'm not sure if i would need it and than i could add it later if the need would arise.
I also replaced tr9 and tr10 with bc141's so it could drive the transformer more comfortably. I'm not sure if i have to change any values surrounding tr9 and tr10 to accomodate this transistor swap. but i'm pretty sure i could just swap them without any change. right? <--- please comment!!!
I'll probably post the layout/schematic tonight since i'm busy doing some mixing right now. i would love to hear any comments on the layout since this will be my first mic pre layout.
greetings,
Thomas
hobiesound
Well-known member
ok here goes, these are the UNVERIFIED schematics and layout for the AM9 minus 15k filter and mute circuit.
I decided to go for a separate gain pcb so i can use a standard MBB lorlin pcb type switch coz i'm cheap!
i changed the output transistor and it's associated current source transistor to bc141 and i added a zobel network and load resistor to the input transformer in case it's needed. I also added a switch for phantom power. The output transformer is mounted offboard so it's not included in the schematic. Make sure to read the datasheet of your output transformer for correct wiring and termination.
ok on to the files:
main pcb
http://www2.hku.nl/~thomas11/DIY/BBC_AM9/ver01-schem.pdf
http://www2.hku.nl/~thomas11/DIY/BBC_AM9/ver01-pcb.pdf
http://www2.hku.nl/~thomas11/DIY/BBC_AM9/ver01-layout.pdf
Gain pcb
http://www2.hku.nl/~thomas11/DIY/BBC_AM9/adapter%20lorlin%20am9%20gain-pcb.pdf
http://www2.hku.nl/~thomas11/DIY/BBC_AM9/adapter%20lorlin%20am9%20gain-schem.pdf
http://www2.hku.nl/~thomas11/DIY/BBC_AM9/adapter%20lorlin%20am9%20gain-layout.pdf
ok like i said this pcb is unverified and untested. I'm going to test it as soon as i get all the parts i've ordered. Untill then please post here if you find any flaws or if you have any suggestions. I made most of this in the wee hours after work and in between mixing and recording my bands ep so double check everything before etching and buying parts etc.!!!
Greetings and cheers,
Thomas
I decided to go for a separate gain pcb so i can use a standard MBB lorlin pcb type switch coz i'm cheap!
i changed the output transistor and it's associated current source transistor to bc141 and i added a zobel network and load resistor to the input transformer in case it's needed. I also added a switch for phantom power. The output transformer is mounted offboard so it's not included in the schematic. Make sure to read the datasheet of your output transformer for correct wiring and termination.
ok on to the files:
main pcb
http://www2.hku.nl/~thomas11/DIY/BBC_AM9/ver01-schem.pdf
http://www2.hku.nl/~thomas11/DIY/BBC_AM9/ver01-pcb.pdf
http://www2.hku.nl/~thomas11/DIY/BBC_AM9/ver01-layout.pdf
Gain pcb
http://www2.hku.nl/~thomas11/DIY/BBC_AM9/adapter%20lorlin%20am9%20gain-pcb.pdf
http://www2.hku.nl/~thomas11/DIY/BBC_AM9/adapter%20lorlin%20am9%20gain-schem.pdf
http://www2.hku.nl/~thomas11/DIY/BBC_AM9/adapter%20lorlin%20am9%20gain-layout.pdf
ok like i said this pcb is unverified and untested. I'm going to test it as soon as i get all the parts i've ordered. Untill then please post here if you find any flaws or if you have any suggestions. I made most of this in the wee hours after work and in between mixing and recording my bands ep so double check everything before etching and buying parts etc.!!!
Greetings and cheers,
Thomas
clintrubber
Well-known member
hobiesound said:ok here goes, these are the UNVERIFIED schematics and layout for the AM9 minus 15k filter and mute circuit.
I decided to go for a separate gain pcb so i can use a standard MBB lorlin pcb type switch coz i'm cheap!
i changed the output transistor and it's associated current source transistor to bd141
Hi Thomas,
Nice action !
You mean BC-types like in your schematic, right ?
Bye,
Peter
hobiesound
Well-known member
tv said:
what do you mean ? isn't the bc141 a npn transistor?
hobiesound
Well-known member
clintrubber said:hobiesound said:ok here goes, these are the UNVERIFIED schematics and layout for the AM9 minus 15k filter and mute circuit.
I decided to go for a separate gain pcb so i can use a standard MBB lorlin pcb type switch coz i'm cheap!
i changed the output transistor and it's associated current source transistor to bd141
Hi Thomas,
Nice action !
You mean BC-types like in your schematic, right ?
Bye,
Peter
Yes totally correct! above post corrected
Very cool! Thanks for sharing!!! ;D
Just wondering a couple things.... what kind of inductors are you using? Not really sure how important they are.
And the "gainboost"... it looks like your connecting an spst between R21 and ground, am I correct here?
Thanks!!
dave
Just wondering a couple things.... what kind of inductors are you using? Not really sure how important they are.
And the "gainboost"... it looks like your connecting an spst between R21 and ground, am I correct here?
Thanks!!
dave
hobiesound
Well-known member
Davo said:
the inductors are of the simple resistor kind. as you can see in the original schematic/layout. Here:
http://www2.hku.nl/~thomas11/DIY/BBC_AM9/Mic_amp_Am9_19_cct.jpg
http://www2.hku.nl/~thomas11/DIY/BBC_AM9/Mic_amp_Am9_19_descript.jpg
Davo said:
You could, but i was intending to just use a jumper.
greetings a happy new year to all!!
Thomas
(btw can i just host the original schematic or should i reference the groupdiy gmail account? )
Transformer question (sorry this is going to be noob-ish)...... the world of transformers is hazy to me, I know that you want roughly the
same impedance between source and load, but not sure how important it is to get an exact match (or what's within the ballpark)
So with the AM9:
Input TX is 1:3.5.... essentially a 600:8K
Output TX is 1:2.... 3K:12K
I'm assuming this is correct as the schemo lists turns ratio and 8k at the input and 12k ouput.
So a modern equivalent usually being around 10k for line impedance, what would be suitable alternatives?
600:10k (1:4) for the input.... seems reasonable... the Lundahl LL1530 is actually a 800:10K 1:3.5
2.5k:10k (1:2) for output..... seems reasonable as well, these are a little bit harder to come by, the Lundahl LL1544A will accomplish
this.... is it important to maintain the output tx's primary at around the 3k?
Any advice is appreciated!!
Thanks
Dave
same impedance between source and load, but not sure how important it is to get an exact match (or what's within the ballpark)
So with the AM9:
Input TX is 1:3.5.... essentially a 600:8K
Output TX is 1:2.... 3K:12K
I'm assuming this is correct as the schemo lists turns ratio and 8k at the input and 12k ouput.
So a modern equivalent usually being around 10k for line impedance, what would be suitable alternatives?
600:10k (1:4) for the input.... seems reasonable... the Lundahl LL1530 is actually a 800:10K 1:3.5
2.5k:10k (1:2) for output..... seems reasonable as well, these are a little bit harder to come by, the Lundahl LL1544A will accomplish
this.... is it important to maintain the output tx's primary at around the 3k?
Any advice is appreciated!!
Thanks
Dave
Davo said:
Seems reasonable. Keep the ratios as close as possible. Regarding when the impedance ratio is expressed as two impedances like you show above, this could be expressed differently by the manufacturer - eg. 5K:20K for a 1:2 line transformer. Remember that transformers only "reflect" / convert impedances from primary to secondary and vice-versa. The mic input transformer should be optimised for 200 - 600 Ohm sources as per the design, and likewise, the output suited for line outputs.
Maybe have a look at what Sowter, Lundahl and Jensen have to offer.
clintrubber
Well-known member
For the cheapo's or at least for a first version of this pre, how about the OEP models ?
Or would that clash with the 'direction' of this pre ?
Regards,
Peter
Or would that clash with the 'direction' of this pre ?
Regards,
Peter
clintrubber said:
I'm sure you could try anything, but that certainly isn't what the original designers would have fitted. If someone wants a dirty/coloured preamp, then I'm sure the FETboy would probably be a good choice.
Also, the 1:2 OEP won't probably function very well as an output transformer here.
clintrubber
Well-known member
rodabod said:
I understand what you mean, thanks for in fact confirming to avoid the OEP-route!
Cheers,
Peter
clintrubber said:
Never say never!
Unless a strange man asks you to pick up a bar of soap in the men's showers.
I guess you can try anything. I just don't see the point in building a broadcast preamp and then fitting transformers which wouldn't meet broadcast specs.
Thanks Rodabod!! I wasn't aware that different manufacturers expressed ratios differently.
So if I were to use the Lundahl ll1530... configured @ 1:7 (200:10K)..... is that too much of a jump in turns ratio?....
I'm thinking in terms of "gain" w/ the turns ratios.... whereas I should probably be thinking just in terms of matching impedances
between source and load, as you stated.
so is 10k too far off from 8k (on the input tx,s secondary) to be usable in this instance?
So if I were to use the Lundahl ll1530... configured @ 1:7 (200:10K)..... is that too much of a jump in turns ratio?....
I'm thinking in terms of "gain" w/ the turns ratios.... whereas I should probably be thinking just in terms of matching impedances
between source and load, as you stated.
so is 10k too far off from 8k (on the input tx,s secondary) to be usable in this instance?
clintrubber
Well-known member
I'll never forget those gloomy showers @ YMCA Chicagorodabod said:
W.r.t. broadcast-specs, I can understand, but more from the BBC-label than neccesarily from this circuit-topology, which is actually not that unlike as used by many other brands (I figure those octal Altec-micpre-plugs won't be considered broadcast-quality). But the devil is in the detail of course, I'm not saying that 'quite alike' topologies sound the same.
Cheers,
Peter
