Ortofon ST-M25 Step-Up Transformer

The Ortofon ST-M25 is a step-up transformer developed specifically for the mono SPU cartridges Ortofon CA/CG 25 Di MkII and CA/CG 65 Di MkII. It raises their very low output voltage by 12 dB — for mono playback through both speakers.

Why the ST-M25?

The classic SPU mono cartridges deliver a particularly low voltage that pushes many inputs to their limits. The ST-M25 is tuned exactly to these cartridges: 12 dB of gain, wideband from 10 Hz to 100 kHz, and it cleanly puts the mono signal onto both channels, so you hear your mono records through your complete stereo system.

What you get

The exact-fit complement for an SPU mono setup — a passive transformer that delivers precisely the gain these cartridges need, with no detours and no active electronics in the signal path.

Who is the ST-M25 suited for — and who isn't it for?

Intended for owners of the SPU mono cartridges CG/CA 25 Di MkII or 65 Di MkII. Straight talk: the ST-M25 isn't designed for other cartridges — if you have a normal stereo MC, the universal ST-7 is the right choice. This one is a specialist tool, not an all-rounder.

Our verdict

The ST-M25 is a niche product in the best sense: the exactly right match for a demanding mono SPU setup. If you run these cartridges, this is the manufacturer-intended solution. At HD Zürisee in Wädenswil, we help you choose and integrate it.

Technical data

• Type: passive step-up transformer (mono)
• Gain: 12 dB (1 kHz)
• Intended use: exclusively for Ortofon SPU CA/CG 25 Di MkII and CA/CG 65 Di MkII (mono)
• Suitable cartridge internal resistance: 5–50 ohms
• Load: > 47 kOhm // 200 pF
• Frequency response: 10 Hz–100 kHz (±1 dB)
• Dimensions (W×H×D): 65 × 39 × 98 mm
• Weight: 410 g

Frequently asked questions

Which cartridges is the ST-M25 intended for?

Exclusively for the Ortofon SPU mono cartridges CA/CG 25 Di MkII and CA/CG 65 Di MkII. The 12 dB boost is tuned exactly to their very low output voltage. The ST-M25 isn't intended for stereo MC cartridges — that's what the universal ST-7 is for.

What does the ST-M25's +12 dB do?

It raises the very low voltage of SPU mono cartridges to a usable level and puts the mono signal onto both channels. This lets you hear mono records at the correct level through both speakers instead of just one. Active electronics aren't needed for this.

Why buy the Ortofon ST-M25 at HD Zürisee?

Because a specialist product like this needs the right advice. We work out with you whether the ST-M25 suits your SPU mono setup, and integrate it cleanly. HD Zürisee in Wädenswil is an authorized Ortofon dealer — call or write to us.

Goes well with

Goes well with: a matching cartridge, a phono preamp, and a high-quality turntable.