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Old 13th December 2012, 11:15 PM   #101
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Maybe a schematic would be easier than so much text.
Here's the basic idea on the K50 fixup. See attachment.
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File Type: gif lm1875t-schematic.gif (5.2 KB, 533 views)
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Last edited by danielwritesbac; 13th December 2012 at 11:19 PM.
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Old 14th December 2012, 01:27 AM   #102
lausar is offline lausar  United States
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Quote:
Originally Posted by danielwritesbac View Post
Maybe a schematic would be easier than so much text.
Here's the basic idea on the K50 fixup. See attachment.
I'm pumped! I need the lowest gain possible without compromising the overall sound. I use low output moving coil cartridges with step-up transformers. The gain add's up quickly between the preamp, SUT's and amplifier. It's quite a balancing act with my speakers. I also need the highest input impedance too.
I have some smps's at work that I can use. They're 75W 24VDC 3.2A
(adjustable) units so I'd like to use these initially until I can build some linear supplies.
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Old 14th December 2012, 05:01 AM   #103
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Quote:
Originally Posted by lausar View Post
I'm pumped! I need the lowest gain possible without compromising the overall sound.
The range of feedback-shunt resistor is:
3.9k 29x gain
4.7k 24x gain (shown on schematic at post 101)
5.6k 21x gain
6.8k 17x gain
7.5k 16x gain
8.2k 14x gain
9.1k 13x gain
10k 12x gain
12k 10x gain
*At some point the amp may need an input stopper resistor (series to in+) to help it restore balance.
**At low gain settings, external compensation may be needed.
Quote:
Originally Posted by lausar View Post
The gain add's up quickly between the preamp, SUT's and amplifier. It's quite a balancing act with my speakers.
If you have trouble with x-max when rocking the house, we can add a soft clipper to block x-max. They're in rough increments and would possibly require some really minor gain fine tuning to match up perfectly.
Quote:
Originally Posted by lausar View Post
I also need the highest input impedance too.
If you add series resistor, it will effectively give the preamp a lighter load and simultaneously "turn down the volume" which might be suitable if the amp has too much gain. Good spot to experiment with a variable resistor.
*Before trying for really light input loads on solid state electronics, most people would be using a buffer instead.
**The amp with the overly light input load will be extra sensitive to grounding and it will need extra attention to RF filtering. See the datasheet for some examples of the RF filtering.
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Last edited by danielwritesbac; 14th December 2012 at 05:10 AM.
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Old 14th December 2012, 10:21 AM   #104
AndrewT is offline AndrewT  Scotland
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If you have too much gain in the system, then maybe D's Buffer suggestion is a good one.

A Power Buffer instead of a Power Amplifier could be the way to go.
i.e. a power amplifier with a voltage gain of 1 (+0dB).
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Old 14th December 2012, 01:49 PM   #105
lausar is offline lausar  United States
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Quote:
Originally Posted by AndrewT View Post
If you have too much gain in the system, then maybe D's Buffer suggestion is a good one.

A Power Buffer instead of a Power Amplifier could be the way to go.
i.e. a power amplifier with a voltage gain of 1 (+0dB).
Another option and a good one. That would put my total system gain at a point where I can open up my volume pot. I can't get past 10 o-clock now.

I need to slow down a bit as I've been cramping nightly to try and catch-up with you guys. This is my winter project along with remodeling my master bathroom so I'll be jumping back and worth between the two.
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Old 14th December 2012, 04:35 PM   #106
lausar is offline lausar  United States
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Quote:
Originally Posted by danielwritesbac View Post
Maybe a schematic would be easier than so much text.
Here's the basic idea on the K50 fixup. See attachment.
How much of the front end I'm I going to retain from the K50. C1 & R3? What values?
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File Type: jpg K50.JPG (42.0 KB, 509 views)
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Old 14th December 2012, 05:20 PM   #107
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Quote:
Originally Posted by lausar
I need to slow down a bit as I've been cramping nightly to try and catch-up with you guys. This is my winter project along with remodeling my master bathroom so I'll be jumping back and worth between the two.
Very fortunate choice of the k50 kit then. They're easy and fun. LM1875 has wide range tolerances (far easier to get "in the ballpark" when the ballpark is gigantic).
Quote:
Originally Posted by lausar
. . . put my total system gain at a point where I can open up my volume pot. I can't get past 10 o-clock now.
I wish I'd heard about this earlier. Excellent news! There's no need to lighten the power amp load until creating a noise floor at the power amp. What you need is a trimmer party. Slightly slower to build, but probably weeks or months faster for desirable end result.

To convert a 3 pin trimmer to a variable resistor, solder the center pin to one of the outside pins--the trimmer now has only 2 pins which can be connected per the schematic. With that schematic, you can adjust during playback, which is a time-saver because it makes audio changes much more obvious and more easily comparable than powering off in-between changes.
Quote:
Originally Posted by lausar
How much of the front end I'm I going to retain from the K50. C1 & R3? What values?
That depends on whatever the preamp/source wants. Since those specs are either missing or very wide range, see attachment. This schematic (attached) is designed to help you find optimal values quickly. If the preamp doesn't have an RC at the output, then you'll want to add RF filter (not shown) at the input of the power amp.
Here's your trimmer party: You listen and play with the dials while the amp quickly designs itself.
Tip: If there are several good choices, aim for that which runs coolest at the output RC's resistor and/or amp heatsink.
Attached Images
File Type: gif LM1875T-TrimmerParty.gif (10.3 KB, 514 views)
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Last edited by danielwritesbac; 14th December 2012 at 05:37 PM.
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Old 15th December 2012, 06:37 PM   #108
lausar is offline lausar  United States
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Default I can't start the party- just yet.

Quote:
Originally Posted by danielwritesbac View Post
Very fortunate choice of the k50 kit then. They're easy and fun. LM1875 has wide range tolerances (far easier to get "in the ballpark" when the ballpark is gigantic).

I wish I'd heard about this earlier. Excellent news! There's no need to lighten the power amp load until creating a noise floor at the power amp. What you need is a trimmer party. Slightly slower to build, but probably weeks or months faster for desirable end result.

To convert a 3 pin trimmer to a variable resistor, solder the center pin to one of the outside pins--the trimmer now has only 2 pins which can be connected per the schematic. With that schematic, you can adjust during playback, which is a time-saver because it makes audio changes much more obvious and more easily comparable than powering off in-between changes.
That depends on whatever the preamp/source wants. Since those specs are either missing or very wide range, see attachment. This schematic (attached) is designed to help you find optimal values quickly. If the preamp doesn't have an RC at the output, then you'll want to add RF filter (not shown) at the input of the power amp.
Here's your trimmer party: You listen and play with the dials while the amp quickly designs itself.
Tip: If there are several good choices, aim for that which runs coolest at the output RC's resistor and/or amp heatsink.
I appreciate the help Dan but I can't see myself adding all the trimmers and finding the initial time to do all that tweaking, at least not this winter. I have to fiddle with my TT motor and build a new set of SUT's this winter too.

Can you get me started with a few values for the input section. I know it won't be optimal until I can find the time.
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Old 15th December 2012, 11:56 PM   #109
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I'm doing my homework...

Understanding and redesigning the input stage of LM1875
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Old 16th December 2012, 12:13 AM   #110
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To answer a question that I received via email:
Click the image to open in full size.
This is "design itself" amp for system symmetry, and every outcome is unique for each application. That is a "significant difference" every time. It can be made commercially and sold for profit.
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